Why wouldn't this work? Instead of multi-classing, allow people to uber-class. Uber-classing is choosing whatever power from whatever class you want. So you make a halfling trickster rogue and you choose a couple powers from the chaos sorcerer. Both rely on CHR and DEX so you're not weakening your character. You don't waste feats or your paragon path to accomlish the uber-class. You have a truly unique character! Think of the combos (goofy names by me)!
Deva Avatar (isolating avenger/ preserving invoker)
Dragonborn Champion (protecting paladin/ inspiring warlord)
Goliath Avalanche (earth warden/ rageblood barbarian)
Shifter longtooth Combat Medic (tempest fighter/ battle cleric)
Tiefling Manipulator (illusionist wizard/ cunning bard)
And that's just a handful off the top of my head! And they wouldn't even have to be optimized. How 'bout a halfling paladin who takes the Bard's vicious mockery for shits and giggles?!
What am I missing? Would this be totaly dumb or unbalanced? Or would it rock ass as much as it seems?!