I like this idea, though I wouldn't give it actual spellcasting. Maybe this could be where a Warlock like flavor comes in, with a ray attack that progressively gets better. Give it rogue skills, good Ref and Will, some built in stealth enablers and go to town. I wouldn't give it actual sneak attack, at least not on top of a magic ray attack that gets up to 10d6.
I'm going to post a build I've always wanted to try, and we'll see how it goes. Yes it multi-classes, but barely enough to be a blip on the radar, and I think it plays like a Rogue throughout.
20 point buy
Halfling Master of Many Styles Monk 2/Rogue 10(I don't think an archetype would make this any better)
Traits: Reactionary (Does any Rogue player NOT take this?)
Monk 1: Crane Style, Stunning Fist, Cautious Fighter, Improved Unarmed Strike
Important equipment: Scimitar, Monk's Robes, anything that helps stealth or perception, or your saves or AC (including boosts to Wisdom and Dex)
Important skills: Stealth, Perception, Perform: Dance (only need the 2 ranks), Acrobatics, the rest are up to you
Speaking of Saves (Without extra equipment)
AC: 17 without equipment, +5 Dodge Bonus when fighting defensively or taking total defense (which you should be doing always) for a typical naked AC of 22, FFAC of 22 or 10(Uncanny Dodge) Touch AC of, you guessed it, 22. Monk's Robes raise those all up by +1, and if you're completely naked you're doing it wrong anyway.
This turned out to be a more defensive build than I originally anticipated as I started writing this, but its one I'd personally love to play. Moving things around so that Two Weapon Fighting can be achieved and you can mix Scimitar and kicks might be fun too, but I'm not sure where I'd place it yet. Also would like to include Improved Initiative, but perhaps a +7 is enough until a spot opens.
EDIT: Changed saves, having forgotten about Halfling save bonuses.
Like everything in D&D/Pathfinder, the game is not played solo, so you have to take the rest of the party into account with any build you play. There are SOME builds that work fine without taking this into consideration, and it has been my personal experience that the Roleplayed personality of those characters tends to reflect that and make them less fun to play with. The Rogue uses the Wizard's intelligence to find out what they're probably going to be up again, relies on the Fighter to give him bonuses to hit and distract the difficult enemies, and otherwise tries to have a tool for every occasion.
The first thing I do making a Rogue is buy anything from the most basic items that seems like it might have a use somewhere down the line. Later in the game, I resort to a Traveler's Any-Tool. Truly, a Rogue's best friend. After that, the Rogue should have one or two (I prefer two) weapons he focuses on, or a double weapon if that's your bag. Beyond those, however, he better have access to some weapons of interesting materials, at least until he's high enough level that at least one of his weapons is a +4.
To all the players I've seen dumping Int on a Rogue: Why? Your own knowledge of a situation shouldn't affect how you go about something in game. If your Rogue doesn't know the answer, he's out of luck. So make him smart enough that he will! I suggest at least a 12-14 here, if you spread your skills out intelligently. Plus, it qualifies you for Combat Expertise, which is considered a feat tax by some, but I've found situations where the party is in a bad spot and the Rogue is enjoying his ability to turtle just a little bit more and avoid those nasty hits. Plus, just look through the lists... there are some fun and promising feats that Combat Expertise is required for.
It is true that some classes can fill a Rogue's role just as well as a Rogue if not perhaps better, but you know what? The Rogue is all about bragging rights! He got the job done, and while he may not have done it with the same flair, the same panache... he wouldn't be screwed in a AMF or zone of Silence, either (At least no more than the purely martial characters in the party).
I know a lot of what I said comes down to money, a finite resource, but, guess what? You're the Rogue! Steal, bribe, cheat, bluff, finagle, and otherwise snatch that money right out of the enemies hands (Or the local townsfolk, or even other party members...). Be a Rogue, do what Rogues do best... improvise!
A good rule of thumb, in my opinion, is that the higher level you are, the more likely you're being watched by someone of importance as a spellcaster especially. by about 10-15 you're becoming at the very least world renowned, and any higher than that and planar beings have started to take notice. What that means, among other things, is that anyone with villainous intent would have a hard time getting up to some of the higher level spells before a group of adventurers gets sent out to stop them.
Factotum 20//MoMS Monk 2/Kensai Magus 18, using Kung Fu Genius for Int to AC for Monk. A literal reading of it would suggest that you wouldn't even need Turn Undead as a feat to use the Opportunistic Piety ability for the Factotum, but that might change depending on DM. Plus, after 8 levels in Factotum, you can full attack to use spell combat, then use 3 inspiration points for an extra standard action for another spell if you need it. Every time you get a feat that you don't need something else for, take Font of Inspiration so you're not burning through your inspiration points as rapidly.
Depending on the makeup of the arena you'll be fighting in, an interesting choice is 20th level Merfolk Aquatic Bloodline Sorcerer. 20 rounds of Control Water with no water source needed, and its dimensions doubled. First round activate this ability, and watch the rest of the combatants drown while you play cat and mouse games with them.
There was a build on these messageboards that I fell in love with when I first saw it. The original was called the Mist Assassin I believe. Levels in Rogue or Ninja and Oracle, with the Waves Mystery and Water Sight. Being able to see in the mist of Obscuring Mist without penalty means you can sneak attack anything that's inside the mist with you that can't see you. Brilliant.
What about making him a mutant frog, and going Aberrant Bloodline Sorcerer focused on touch attacks? by 3rd level, you're making touch attacks at 15 feet with your tongue, which ends up going as far as 25 feet by 17th level if your game goes that far. The Acidic Ray ability even feels thematic in a way.
I agree with the Ultimate Campaign comment. Use the tables in that, as many as possible, to generate a completely random table. Take it a step further and create a table using the races and classes and roll on that. Play a one shot with these randomly generated characters that the players are forced to figure out the motivations for, to play as if they'd created them from scratch.
I don't profess to be a master at this sort of thing, but I looked over the Marshal in response to seeing this thread, and these are my initial findings.
Advance seems to me to be the best Marshal's Order of the three, though you would not be wasting Path abilities on Additional Order for the other two I don't believe. The ability to get extra actions is just killer, no matter what, and the versatility of move actions could make this amazing in the right party.
Beyond that, a lot of it really depends on the makeup of the rest of your party. If they're into high crit range weapons, and if you are too, then Directed Assault is for you.
Deadly Guidance plus Distracting Assailant is pretty brutal, especially if you've already got someone with a source of sneak attack to add on top of it.
Inspire Minions would be amazing if you or someone happened to take Leadership, but really kind of useless otherwise.
Lightning Performance is a must, and Inspired Defense helps to shore up any bad saves your party might have.
Remember what I said about Leadership? Loyalty gives it to you. if you go down that path, definitely take this, even if you already are taking Leadership with a feat for the added benefit.
Potent Message has some wonderful roleplay potential, depending on how WotR plays out.
Resurging Words works great right before a final push or last stand.
All of that is just the 1st tier abilities, and just from a single pass through it.
There are likely to be bones of some kind lying around. Find a devil that uses weapons, smite and take it out, then take its weapon. No matter what that weapon is, since you're 14th level it'll be treated as good aligned, even when you're not smiting. That'll be a plus. If you chose the mount option of the divine bond then you've got transportation. I'd start looking for a portal out.
I'm going to throw something out of left field a bit...
Savage Skald Bard, possibly mixed with Barbarian or Fighter. Mix in a little Catch Off-Guard and use that mutton as a weapon! As for race, Half-Orc would make a lot of sense here, and to showcase his muddled line you could throw in an Eldritch Heritage feat or two.
A Maneuver Master Monk focused on Grappling could be fun, or a Tetori if you don't mind some of the more mystical qualities of it. Stunning Pin, Chokehold and Neckbreaker are probably most thematic here. The stealth wouldn't need to be multiclassed at all, just put ranks in it every level, have a good dex and maybe buy an item or two that helps it, and Skill Focus if you want the extra oomph.
The gun part would be harder, though since you're not looking to flurry with a weapon then a level (or 5) of Gunslinger wouldn't be hugely detrimental.
If you want to truly play Emperor Palpatine, then Enchantment/Necromancy focused Wizard is the way to go, most likely Enchantment specialist with Necromancy for some of the darker Force powers, and at least Shocking Grasp. Yet another problem of the prequel movies, the man should have never drawn a lightsaber. It wasn't his strength. He plays people against each other like a chess board.
EDIT: And unlike some wizard builds you see, Charisma is at least moderately important for his ability to charm people with or without The Force. Strength would be your dump stat, if you need one.
You are the DM so some prerogative could be taken here, and for Earth I think you should relax the requirements for Stonelord and go with a Dwarf-blooded Oread. (but keep Stonelord, that's an awesome idea)
Fire should be anger and hatred... an Ifrit Barbarian/Crossblooded Draconic Elemental (Fire) Sorcerer/Red Dragon Disciple, with the Elemental Rage (Fire) line and as many fun fire based spells as you can get.
Air is a Ninja focused on stealth and invisibility, maybe a Sylph.
Water is your healer, am Undine Waves Mystery Oracle. If you want them to be a "warrior" as well, multiclass Rogue or Ninja and use the Water Sight revelation and Obscuring Mist to set up sneak attack as often as you'd like.
Further looking into this, Antipathy on the tower is a good idea, especially if the invading army is of a particular kind, or aligned a particular way.
Elemental Swarm makes for a good attack troop, to go after the bulk of the enemy army.
Shambler plus Call Lightning Storm could be a lot of fun.
If your city is surrounded by trees or plants of any significant size, Siege of Trees and the Greater version would make for good defenses of the city itself.
Summon Elder Worm, let the Purple Worm burrow its way under the city and out to the advancing army, wreaking havoc on their ranks.
The idea of Energy Siege Shot made me think... being an NPC, and fulfilling a very specific role, this might be a perfect time for a Siege Mage. Not an archetype that seems worth it most of the time, but if you had your tower and city protected by cannons or the like, you could do some fun things with that.
Shades for something like a quasi-real Ramparts, create a barrier that you can still cast/see through but that hides your position.
A Mass Hold Monster trap, or better yet, Overwhelming Presence trap.
Gate, with some friends you've met along the way ready to come help.
Globe of Invulnerability, always useful.
Summon Monster IX
Spellbane if you know the spells you want to stop.
2 levels of Paladin is nice for smite evil and Cha to saves, but you're NG making that harder.
Mysterious Stranger Gunslinger for the Cha synergy, use a pistol for your first attack, drop it on a weapon cord and you've got a hand free for your scimitar which you can Dervish Dance, or if you don't go the DD route then straight up TWF with the pistol and sword.
What about an Infernal Bloodline Sorcerer/Dual Cursed Oracle/Mystic Theurge? Infernal seems more in line with the deal-making and lawful aspects of his character, and you go dual cursed to make Wasting the curse that doesn't progress, and Legalistic to show his inability to step away from the chance to make a contract.
This screams Monk, to me. How I'd build it though, let's see...
using 3 level up ability score increases on Dex Con and Wis, and the other two on strength to end with:
I'd replace Earth Affinity with Crystalline Form, and Energy Resistance with Granite Skin, and Magic Stone with Treacherous Earth. I'd take Crane Style/Crane Wing at the very least, maybe Shaitan Style and Shaitan Skin if you can fit it in to regain the acid resistance and more. Other than that I might focus on trip to keep people in the difficult terrain created by Treacherous Earth.
A lot of it depends on the campaign you're going to be in, and what that world is like. I could easily see someone who is a dungeon delver (class or theme, either way), who explores ancient tombs and dungeons, and takes things that are there because they're not being used by anyone, anymore. If they're given a mission to take a specific thing for a particular person/entity, they wouldn't go back on that deal but anything they weren't told to bring to them would be fair game.
At the same time, I've also played in a game where the recognized government included a thieves' guild. It was lawful in as much as there were codes and laws about when and what you could do, but thieves were perfectly fine doing what they do, as long as they had the backing of the guild.
Basically, what you want is to find someone who is licensed/authorized to do what they do, because they have a particular set of skills and those skills are useful to someone high up.
Waves Mystery Oracle/Rogue (or Ninja, but less thematic for pirate campaign). Take Deaf Curse, and Water Sight revelation and go ranged, dropping an Obscuring Mist on an enemy ship right before boarding. Bonus if you can talk at least a portion of the rest of the party to take classes that get something similiar to Water Sight.
For Race, go Half Elf to keep favored class bonuses going the whole way, and maybe to pick up longbow proficiency
Gaze of Flames will most likely be your best friend. Through up a fog type spell and pelt your enemy with fire from afar when they can't even see you. Waves mystery does this better but you can still have your fun.
With Efreeti Magic you can make use of that reduce person on yourself and be a hard to hit little ball of flaming goodness.
I built one of these for an arena style match, and had a blast with it (pun intended). Built basically like a blasting sorcerer.
War: Battle Herald or Oracle of Battle, focusing on instilling the urge to kill in everything and everyone around you.
Famine: Witch using Putrefy Food and Drink, Ray of Sickening, Feast of Ashes, and the Blight Hex
Pestilence: Antipaladin, focusing purely and solely on diseases and spreading them
Death: This one is where I would differ a good bit from the rest. The Bible verse says "They were given power over a fourth of the earth to kill by sword, famine, and plague, and by the wild beasts of the earth". I've always read that to be describing the four horsemen, and sword is obviously War, famine is Famine, plague is Pestilence... so Death would kill by the wild beasts of the earth, or by being a summon-focused character, either a Conjuration Wizard or a Summoner.