If you could mesh up games or aspects of games...

If I could instantly conjure up a game with combined characteristics of other games I would combine horse back combat from Legend of Zelda: Twilight Princess (and maybe apply it to other animals), with the choices from Mass Effect, the details and scale of Elders Scrolls: Oblivion, ... and put them all together for the best action-adventure-rpg game ever made.

Replace the horse combat with the horse combat of Mount&Blade, and maybe this thread won't get spammed with flamers.
 
Maybe.

" Replace the horse combat with the horse combat of Mount&Blade, and maybe this thread won't get spammed with flamers.
 
Maybe. "

COD:MW2 and Mirror's Edge. DUDES WALLRUNNING AROUND N SHOOTIN IN MULTIPLAYER ROFL 
 
No seriously, it could be pretty neat.

Shadow of the Colossus with Uncharted 2 graphics and modern technology on current-gen system

Mount and Blade combat + Elder Scrolls size and even world + A few more little RPG twists like those in Dragon Age = Great RPG game.
 
For FPS I would say MW2 weapons and perks and general level design + BF2 vehicles and level size + really good destructability, probably like that in BC2

" COD:MW2 and Mirror's Edge. DUDES WALLRUNNING AROUND N SHOOTIN IN MULTIPLAYER ROFL  No seriously, it could be pretty neat. "

Red Alert 3, but more like Red Alert 2

" Shadow of the Colossus with Uncharted 2 graphics and modern technology on current-gen system "

Combine the shooting mechanic of Borderlands to the quests of Fallout 3 with the graphics of Uncharted 2 and the rotary wheel response hub of Mass Effect.  Also DOA breast physics applied to every game.

I have quite a few I would like to see actually: 
  
Third person shooter with the gameplay feel of Gears with all the RPG mechanics that Borderlands had. I would prefer that it had a straightforward plot with some sort of central hub you could do side missions from kind of like FF7 Crisis Core had. 
 
Pokemon except with Fallout "feel" (open world/speech options). Being able to go anywhere you wanted and do what you wanted has been my dream for the Pokemon series for years. The universe has all these sort of "roles" besides being a trainer and I have always wanted to experience in an open world setting.  
 
Crackdown with gameplay (fighting and character customization) of Kingdom Hearts (in particular number 2 and the newest DS release 358/2). Gliding along side jumping ultra high would be totally bad ass.
 
One of my bigger dreams is tossing The Sims, Persona, Animal Crossing, Grand Theft Auto, Harvest Moon, and (gameplay parts of) Final Fantasy XII into a blender and seeing what the end result would be like. I think I would be in love if pulled off correctly.

The combat of bad company combined with a slightly tweaked geo-mod2.0 engine (red faction)

"  One of my bigger dreams is tossing The Sims, Persona, Animal Crossing, Grand Theft Auto, Harvest Moon, and (gameplay parts of) Final Fantasy XII into a blender and seeing what the end result would be like. I think I would be in love if pulled off correctly. "

Halo and Call Of Duty........

The amazingly smooth and well done animations of Uncharted 2 with every game ever! I so want that game, and a PS3.