It depends what you mean by the simplicity of 2E. For instance, it had the most annoying system for saves, but at the same time, if a DM wanted to make a fighter to oppose the players, he could do it in less than ten minutes without looking at the books. I recall a number of things working like that, actually. Really quick and simple to make
Although personally, I'd hate to play a game where, if I chose "fighter", that was it, I was seriously the same as any other fighter. It seems tricky - what's good for a DM (just rapidly making something with a couple of choices) is bad for a player (lack of options). One of the things I liked about 3E was that you could generally make the character as you pictured them - sometimes you had to dive through books for the right class or feat, sometimes you had to make your own up (and for the most part, that's not too hard if you don't ask for criticism on the WotC boards), and sometimes you had to start at a higher level, but it could be done.
So my guess would be to make a nice, simple core system, one that's easy to utilise, and then to allow a nice variety of options as you go along - but for the options to lack too much complexity (but to still be different). At certain points, you'd simply get non-combat abilities, and these could not be traded for more combat ones (and vice versa). There's nothing stopping us from making as many social abilities as combat ones (and then having exploration type ones as well), and gaining both at the same rate, with politics and social scenes having as many interesting abilities as face-stabbing.
Likewise, if something isn't too important, we could leave it out. After all, movement and ranges aren't that big an issue - if you're not using minis, you could just say "melee", "short range", "long range" and "too far". Obviously, attacks of opportunity would go out the window (do I hear a cheer from the audience?)
It could indeed be done.
Oh, and apparently a well-built cleric can do everyone else's job and wins the game in 4E. Some things never change.