Real life: the game rpg
I have two suggestions.
A) if you are up to it, you can easily modify dnd to achieve many things. In this case, raw power can be split from level so you can hand out lots more levels without the raw power becoming a problem. But this does require a designer's mindset and understanding the rules in a way most players (and too many gms) don't.
B) an easier option is to use Basic D20, a series of alternative rules for d20. Some of these can be used to alter the core dnd rules to better fit what you are wanting to do.
Both options build on you player's familiarity with d20 and has d20's moderate but not overwhelming crunch (cause, if your players are like me, Hero could be far too much crunch, gut even if it isn't, tackling a specific and unusual campaign concept is a bad time to learn a new system, as that detracts from the campaign moments)