I already have a 10d6 fireball in my pants, thank you very much.
I, too, have come up with PC concepts that cannot be designed in 3.xEd. I'm not even talking about truly esoteric stuff that I can only do in HERO, but stuff I have done in past editions of D&D. My 2Ed Minotaur Ftr/Mage and my Finnish legend based warrior/cleric simply cannot be translated into the current game.
Heck, I'm running a Spellsword right now who is based on Indiana Jones...very clunky in the current rule-set.
Getting rid of classes to make it "pick whatever you want" would make it not D&D anymore.
There are still ways to make the game more flexible than it is without ditching the class system.
If you make all options available to all classes, but give certain classes a "discount" on acquiring certain options, you'll keep the class system, but increase flexibility. See 2Ed's Players Option ruleset.
In 3.X (or 4Ed, if you will), that would mean you'd have multiple ways of achieving a magic-using warrior. The warrior would be able to pick up fighting-related abilities "cheap," but would find picking up spellcasting "expensive," while a spellcaster could pick up magic-related abilities "cheap" while combat skills would be "expensive." Both might find rogue type skills to be "moderately priced."
This doesn't have to be a point based system.
If, for example, everything were to be reduced to Feats, Warriors would pick up warrior feats at a cost of 1 Feat slot per Feat, but might have to allocate 3 feat slots to pick up moderate arcane spellcasting ability (say, like a bard's) or 6 feat slots to gain full arcane spellcasting ability (like a sorcerer or wizard).
This change would neccessitate only that PCs get more feats to spend.