I that case, I probably would look deeper in the posibilities offered by variant classe levels.Also a key thing. Instead of every Monk getting Ability X at 10th level, different Monks could branch out in different directions *without* having to abandon the Monk class to take a PrC that has as it's first line of text 'advances the following Monk abilities...'
'Cause that's just lame. *Far* too many PrCs, IMO, are, at best, three to five levels long, but are stretched out into 10 level PrCs by 'filler' levels that just advance abilities from your previous class. If a level of a PrC advances your class abilities, why the heck isn't it a level in the previous class?
I'm currently designing a class system for my campaign setting, and I think I've tried and discarded almost all options that people have thought up for that.
Generic base classes, prestigious classes, skeleton base classes with multiple paths, 10 level prestige classes, 5 level prestige classes, feat customization, variant levels, selective class features, paragon paths, ... you name it. ^^It really starts to get difficult when you try to make a system that is simple. ;)
I hate to admit it, but I kind of think 4.0s Paragon Paths are about the most appealing viriant to me. (Maybe I misread the system when I flipped through and dismissed 4.0, but that's how I think it's meant):
All classes begin fairly generic for the first couple of levels and at some point you select one of many specialized paths you wan't your character to take. I think it's a quite good way to represent the speciialization into one type of advanced methods and techniques.
The downside is, that though you could make "secret" paths that can only be taken by initiated members of a secretive society, you would be unable to take it up when you already have picked another path. And feat chains you could pick up at any other level seem far to complex to manage to my tastes.
I think the prestige class system really has its place, when it comes to things like that. But In that case, it's probably right, that 5 levels are completely sufficient. A wizard who is initiated into the secret art of demonology and choses to completely master this art should still be foremostly a wizard. 5 levels of wizards should not be the entry test to begin with his real major class. At least in my oppinion.
Maybe make it "Advanced paths", not much unlike Rogue Special abilities, for general and generic specializations, and 5 level Prestige Classes for specializations unavailable to uninitiated characters? Has the nice advantae that you can very easily add new Advanced Paths later.
Fighter paths: Archer, Duelist, Knight
Cleric: Warpriest, Paladin, Exorcist
Rogue paths: Swashbuckler, Assassin, Spy
Wizard: Necromancer, Seer, Warlock