Just goes to show how diverse people's opinions can be. The rules for class/levels with demi-humans was in my opinion one of the worst things
I like the idea of them. In a 3e game, if they had it. I envision it like this:
All PCs' levels max out at 20 (I'm not a fan of epic). If demi's had limits on their classes. They would have to fill the rest with something else. Maybe something similar.
I'm not a fan of PrC's, but I am with character class options. Maybe if PrC's were 3 or 5 levels then they'd be fine. I like the idea of a advanced class. So maybe dwarves have a fighter limit of 15. They have a barbarian limit of 5 (since they're a lawful race). A class option for barbarians might be "dwarven defender". So you might take 5 levels of defender to fill out your 20. Or 5 levels of bard.