Last one for a while, I promise!
This is an attempt to make a mage/thief base class, drawing on inspiration from the rogue, arcane trickster, beguiler and spellthief. I'm pretty happy with it but I'd like to see if you guys can see anything wrong with it which I should fine-tune. I was considering giving it its own spell list but that's never popular and to be honest I don't see anything gamebreaking by him having the sorcerer/wizard spell list.
Alignment: Any
HD/Base attack: d8/Medium
Good saves: Reflex/Will
Class Skills: Acrobatics (Dex), Appraise (Int), Bluff (Cha), Climb (Str), Craft (Int), Diplomacy (Cha), Disable device (Dex), Disguise (Cha), Escape artist (Dex), Knowledge (Int), Perception (Wis), Profession (Wis), Sense motive (Wis), Sleight of hand (Dex), Spellcraft (Int), Swim (Str), Use Magic Device (Cha).
Skill Ranks per Level: 4 + Int modifier.
Weapon and Armor Proficiency: Conmages are proficient with all simple weapons, plus the hand crossbow, rapier, sap, shortbow, and short sword. They are proficient with light armor, but not with shields. A conmage can cast conmage spells while wearing light armor without incurring the normal arcane spell failure chance. Like any other arcane spellcaster, a conmage wearing medium or heavy armor or wielding a shield incurs a chance of arcane spell failure if the spell in question has a somatic component. A multiclass conmage still incurs the normal arcane spell failure chance for arcane spells received from other classes.
Spells: A conmage casts arcane spells drawn from the sorcerer/wizard spell list. He can cast any spell he knows without preparing it ahead of time. To learn or cast a spell, a conmage must have a Charisma score equal to at least 10 + the spell level. The Difficulty Class for a saving throw against a conmage’s spell is 10 + the spell level + the conmage’s Charisma modifier.
Like other spellcasters, a conmage can cast only a certain number of spells of each spell level per day. His base daily spell allotment is given on Table: Conmage. In addition, he receives bonus spells per day if he has a high Charisma score.
The conmage’s selection of spells is extremely limited. A conmage begins play knowing four 0-level spells and two 1st-level spells of the conmage’s choice. At each new conmage level, he gains one or more new spells, as indicated on Table: Conmage Spells Known. (Unlike spells per day, the number of spells a conmage knows is not affected by his Charisma score.
Upon reaching 5th level, and at every third conmage level after that (8th, 11th, and so on), a conmage can choose to learn a new spell in place of one he already knows. In effect, the conmage “loses” the old spell in exchange for the new one. The new spell’s level must be the same as that of the spell being exchanged, and it must be at least one level lower than the highest-level conmage spell the conmage can cast. A conmage may swap only a single spell at any given level, and must choose whether or not to swap the spell at the same time that he gains new spells known for the level.
A conmage need not prepare his spells in advance. He can cast any spell he knows at any time, assuming he has not yet used up his allotment of spells per day for the spell’s level.
For spells per day and spells known, just look at the link above (although it's the exact same as the bard).
Level progression
1st: Cantrips, cloaked casting +1, ranged ledgermain, trapfinding
2nd: Sneak attack 1d6
3rd: Evasion
4th: Silent spell
5th: Cloaked casting +2
6th: Sneak attack 2d6
7th: Still spell
8th: Tricky spells 2/day
9th: Dispelling strike
10th: Cloaked casting +3
11th: Improved evasion
12th: Tricky spells 4/day
13th: Pilfer magic
14th: Sneak attack 4d6
15th: Cloaked casting +4
16th: Tricky spells 6/day
17th: Vanish
18th: Sneak attack 5d6
19th: Surprise spells
20th: Cloaked casting +5, master trickster
Abilities:
Cantrips (Sp): Conmages learn a number of cantrips, or 0-level spells, as noted on Table: Conmage Spells Known under “Spells Known.” These spells are cast like any other spell, but they do not consume any slots and may be used again.
Cloaked casting (Ex): At 1st level, whenever you cast a spell on a foe who would be denied its dexterity bonus to AC, you gain a +1 bonus to your spell‘s DC and caster level checks to overcome spell resistance. At 5th level, and every five levels thereafter, this bonus increases by +1.
Ranged ledgermain (Su): A conmage can use Disable Device and Sleight of Hand at a range of 30 feet. Working at a distance increases the normal skill check DC by 5, and a conmage cannot take 10 on this check. Any object to be manipulated must weigh 5 pounds or less. He can only use this ability if she has at least 1 rank in the skill being used.
Trapfinding: A conmage adds 1/2 his level to Perception skill checks made to locate traps and to Disable Device skill checks (minimum +1). A conmage can use Disable Device to disarm magic traps.
Sneak attack: At 2nd level, a conmage gains the ability to make sneak attacks as a rogue for 1d6 extra damage. Every four levels thereafter, the sneak attack damage improves by 1d6.
Evasion (Ex): At 3rd level and higher, a conmage can avoid even magical and unusual attacks with great agility. If he makes a successful Reflex saving throw against an attack that normally deals half damage on a successful save, he instead takes no damage. Evasion can be used only if the conmage is wearing light armor or no armor. A helpless conmage does not gain the benefit of evasion.
Silent spell: A conmage gains Silent spell as a bonus feat at 4th level.
Still spell: A conmage gains Still spell as a bonus feat at 6th level.
Tricky spells (Su): Starting at 8th level, a conmage can apply the benefits of silent spell and still spell on a spell he casts without increasing its casting time or spell level. He can use this ability twice per day at 8th level and two additional times every four levels thereafter. The conmage decides to use this ability at the time of casting.
Dispelling Attack (Su): At 9th level opponents that are dealt sneak attack damage by a conmage are affected by a targeted dispel magic, targeting the lowest-level spell effect active on the target. The caster level for this ability is equal to the conmage's level.
Pilfer magic (Su): At 13th level, whenver the conmage successfully dispels a magic effect from an opponent he gains the benefit of that effect for a number of rounds equal to his charisma bonus, or for the remaining rounds the spell would have lasted (whichever is lower). A conmage could only pilfer magic which he is a legible target for.
Vanish (Su): At 17th level, whenever the conmage makes a reflex saving throw to avoid damage, he can turn invisible as an immediate action. He can stay invisible in this way for a number of rounds per day equal to his charisma bonus. These rounds must not be consecutive.
Surprise Spells (Su): At 19th level, a conmage can add her sneak attack damage to any spell that deals damage, if the targets are flat-footed. This additional damage only applies to spells that deal hit point damage, and the additional damage is of the same type as the spell. If the spell allows a saving throw to negate or halve the damage, it also negates or halves the sneak attack damage.
Master trickster (Su): At 20th level, a conmage becomes a master trickster. He can use the tricky spells ability at will. Furhtermore, each time he deals sneak attack damage, the target must make a will save or be confused (as Confusion). The DC of this is 10 + 1/2 the conmage's level + the conmage's charisma modifier.