Back in 1st/2nd edition, I had two additional base classes in my games. One was called the sorceror (a name that has been subsequently used by 3rd edition). It was essentially a secular mage/priest open to humans, with considerably different fluff text. The other was called the rogue (once again the name was subsequently swallowed), and it was essentially a fighter/thief, once again open to humans.
In 1st/2nd edition, these classes were balanced primarily by having extremely onerous experience tables (the sorceror had an xp table that was 2x the fighter table, giving it a post 9th level progression of 500k xp per level, the rogue's table was 2x cleric). This tended, interestingly enough, to lead to results very similar to 3rd edition/Pathfinder---the sorceror tended to run about 3/4 the level of a wizard---it would hit 15th level about the same time a magic user hit 20th.
The fluff for the class was that at the bottom, the use of magic is the coercion of reality by the will (Reality and Will would both be capitalized were this a formal article). Magic users use fancy formulae and rotes refined over centuries of study to focus their will. Priests use faith and some divine assistance (although back in those days, a priest could employ level 1 and 2 spells even if their god was dead, or even if he'd never actually existed, so long as his faith was undaunted). The sorceror, although since that name now has baggage---we'll call him a Primal Mage---is the result of extremely advanced magical research, probably a discovery out at around DC60 on spellcraft. Three things can considerably reduce this difficulty---the first being some understanding of psionics, which deals more directly in the Will. The second is the realization that divine magic can still be employed to a limited degree by clerics that are 'out of supply' (e.g., in a parallel multiverse wherein their god is not present) or followers of a Dead God---this gives the clue that clerics have always been doing a little of this all along. The third is if a god, godling, or demigod actually explains a little of the process of wielding primal power---this is most likely in the case of either a very very newly ascended Power or one who is giving a peek behind the Curtain to a candidate to immortality.
Simply put, a Primal Mage accomplishes magic largely through the Will. His magic observes no distinction between the arcane and the divine. He has a spellbook like a wizard, although the spells contained on it may be drawn from either divine or arcane lists (they may be drawn from the cleric and wizard lists only though).
Like a wizard, they gain 4 spells per level that they advance to add to their spell book. One of these spells is so well known that they need not their spell book to prepare it (as per spell mastery).
Their spell progression table looks like this:
1st 3 1
2nd 6 2
3rd 8 4
4th 8 4 2
5th 8 5 3
6th 8 6 4
7th 8 6 4 2
8th 8 6 6 4
9th 8 8 6 4 2
10th 8 8 6 5 3
11th 8 8 6 6 4
12th 8 8 8 6 4 2
13th 8 8 8 6 5 3
14th 8 8 8 6 6 4
15th 8 8 8 8 6 4 2
16th 8 8 8 8 6 6 4
17th 8 8 8 8 7 6 4
18th 8 8 8 8 8 6 4 2
19th 8 8 8 8 8 6 6 4
20th 8 8 8 8 8 8 6 4 2
(Gains their 8th level spells at 20th level, much like a mystic theurge with the 5 wizard/5cleric/10 MT build would). The rule of thumb is 3 levels of cleric and wizard casting per 4 levels of Primal mage, with the spell progression somewhat interpolated and the 1st and 2nd level progressions modified a little to make the class a less tempting dip.
At level 1, the Primal mage gains bonus spells on intelligence only, gains access to cantrips, and gains scribe scroll
At level 2, the primal mage gains access to orisons, and gains bonus spells based on wisdom as well.
(special note---there are a few spells, mostly commune and miracle, for which a close alliance with a deity is presumed in the casting. A primal mage can not make this assumption by default and thereby can't actually cast these spells unless the GM deems that a sufficient alliance level exists between the Primal Mage and the deity in question).
At this point, the class would be mostly finished as a conversion from 1st/2nd edition, but we'd have to add some minor and fluff powers to balance it with respect to other Pathfinder classes and the Mystic theurge.
The primal mage gets to use his spells totally indifferently between mage/cleric lists, because he only has one list. This is a bit better than the mystic theurge's prestige class ability. However, the Mystic theurge gets automatic access to all clerical spells, which the Primal mage does not. Also, the Primal mage doesn't get domain abilities or domain bonus spells or channeling. Also, the primal mage doesn't get arcane bond or familiar, not may he be a specialist of any sort, nor does he get the bonus of the universalist. The Primal mage's saving throws and BAB are both on the wizard table, which is slightly worse than most MT's, and he gets weapon and armor proficiencies similar to a wizard's. Accordingly, we probably ought to add some capabilities. One thing to note though, since this isn't a prestige class anymore, the Primal mage will usually benefit from a favored class bonus.
Any thoughts on what to add or feedback?
In 1st/2nd edition, the version of this class was generally described by its players and party members as lacking the punch of a pure magic user, particularly at high levels of experience points, but having tremendous flexibility and staying power. It had no 'hazing levels', and this version doesn't have such either. It doesn't have the ability to get 9th level spells in a 20 level progression though, but it would have 9th level capability around level 23 or so in any extended progression. It also doesn't require any legacy feats or traits to have a full casting level. It is not quite as MAD as a mystic theurge, since all its DC's are based on INT, Wisdom is just a pure bonus for them.