How about just Fighter/Warrior & Caster/Adept? These are, IMHO, the only two classes you really need.
If you make BAB, Saves, & spell casting skill-based (or skill-&-feat based), then you only need one class.
After talking over the options for my own d20 variant with my group, that was the direction they seemed most interested in. (I still lost interest in developing it after a few weeks, though...) Of course, when going that direction, you need to be able to answer the question: Why aren't I just using BRP, Gurps, &c.?