I've been tinkering with the concept of turning the game Lords of Magic into an actual Pathfinder game. This would be a difficult conversion because of the magic system and how it's tied into the alignments of the world.
See, Lords of Magic has Faith system that would be easy to convert into alignment. Namely, there are eight faiths: the Elemental faiths (Fire/Earth/Air/Water) and the Arcane faiths (Life, Order, Chaos, Death). Each of these is allies with certain faiths and enemies with others (their opposites). This makes designing the alignment system really easy: you have eight alignments (one for each faith) and unaligned characters (which are generally going to be non-magical NPC monsters).
However, spellcasting is directly tied to those elements. Earth mages use Earth magic, and ONLY Earth magic. Chaos mages use ONLY chaos magic. The list goes on and on. Also, Earth mages may have Protection From Air as a spell, but they probably don't have Protection From Death because Death is their ally.
Given all that, how would you go about designing spellcasters for this setting? They'd all be low BAB spontaneous casters with full spellcasting progression, and they'd have to use spells that only come from their element. Scrolls become a unique commodity because they allow mages to cast spells they normally could not (an Earth Mage CAN use an Air Scroll), so that should be taken into account. They'd probably also learn spells like a Wizard does: by buying the spell and scribing it into their spellbook.
Any thoughts on how to make this work? I dread the prospect of sifting through every spell in the Core and APG and Ultimate Magic to find spells that fit each type (and then retool some of them to fit specific Elements / Arcanum) , but if that's my only real option that's what I'll do.
I recommend Spheres of Power.
Look at the comment section and the review (or ask here) for details but essentially a new magic system where magic is tightly divided into 20 subdivisions called spheres. Its also made to be relatively customizable to however the GM wants magic to work. Its very possible to restrict different spheres to different alignment or restrict two spheres from being used at the same time. It also comes with some classes three of which are fairly customizable. Each spherecaster gets a 'tradition' that is pretty much a list of drawbacks and benefits that caster gets. Although there are drawbacks and benefits listed it is easy to make a tradition that says, for example, "You cannot gain the Death Sphere, You must be good aligned" and have it apply to players that take the Life Sphere. And in exchange give them bonus spell points.
Spheres of Power takes a very old concept called, of course, Spheres of Influence (basically, how Clerics worked back in the day, before the age of Domains), and prescribes them to all spellcasters.
It's a really neat concept, and in many ways fits "thematic" casters in literature a bit more than the "omni-mage" Wizard/Sorc/Arcanist spell list typically does.
It's not exactly something for beginners, however, so be sure you're familiar with the base game before applying something as radical as SOP.
It's not exactly something for beginners, however, so be sure you're familiar with the base game before applying something as radical as SOP.
You need to be pretty familiar with the core rules to finaggle out inevitable corner cases and points where the game expects you to be capable of dealing with something that spellcasters normally deal with easily, (detect magic is a big hole) but I'd argue that Spheres are way more intuitive for beginners than Spells. When I first started playing Pathfinder, spellcasting was one of the most counter intuitive parts of the game and added to that I had so many spells to go through just to figure out what I wanted to do, and too many spells seem like (or are) trap options making new player spell selection so painful. And that's just with the core rulebook.
With spheres you have 20 magic subdivisions that mostly do what they say on the box and each one has 2-4 pages of talents. Plus they follow the talent/feat/package paradigm as the rest of the game. (Although since spheres are the only prerequisites its actually cleaner.)