Lets say Blizzard decided to do a true sequel to World of Warcraft and built a new engine from scratch. Maybe they even released Warcraft 4 to move the timeline and change the world drastically. What would you want from a completely new game. I'll list my top 3.
Larger zones
I want to be able to get lost in the forests of Ashenvale for days before I find my way out. Imagine a Stormwind the size of Liberty City is GTA IV
Better looting system
If I kill a wolf he shouldn't be keeping boots in his stomach. Let crafters create the basic gear we need and allow us to explore and find reagents to increase there usefulness. The whole idea of a boss dropping items he is obviously not carrying is silly.
Physics Engine
I'd like the idea that if I get hit by a large create I don't always have the same animation after I get hit. If I get hit into a tree I should come off it at a angle. Make the game more dynamic and fun.