I understand that it's nearly impossible to implement at this point in time because of technological limitations, but I think something me personally (and probably just about everyone else) is waiting for is a game that offers the ability to destroy just about every article and material in the game such as objects, characters & walls.
Completely destructable environments!! Sure, a game can look really impressive.. (just look at the demonstration of UnrealEngine3) but can it offer a world of destructable and dynamically changable materials?
The kind of processing power would be awesome to produce dynamically changing geometry on a constant basis: i.e. bullets actually chipping away at walls and rockets blowing holes through buildings and eventually allowing the physics to topple the building! OH GOD, I wish such a game existed!
Actually...if someone would devise a system that could be moduled into games... like a system that would map how objects are created and are destroyed, then maybe it would be a little easier on the developers... A system much like the implementation of Havoc in modern games for physics!