What kinda surprised me in 3E, and some of my players, is that they created so many "warrior-mage" prestige or alternate base classes, and nearly all get their own spell progression and spell list. Instead of simply going ahead and give them a level of "arcane caster of choice" every other level or so, making it easier to be compatible with later added levels of the same caster class. Like taking a fighter, D8 instead of D10, medium BAB, adding a level of wizard every few levels (only the spell casting, though) and the ability to ignore armor spell failure chances every few levels, and you're done. Maybe add bonus feats that can be either fighter or metamagic feats. If a character with that class wants to take more wizard levels later, they can stack with the already existing caster levels, since those are wizard levels. So taking one level of wizard while already having 6 wizard caster levels lets the character cast like a 7th level wizard.