I would do an Eldar action RPG. You start as a guardian and choose your aspect, eventually becoming an exarch. As a guardian, there would be levels centered around you ability to use certain vehicles, like warwalkers, jetbikes, various tanks, ect. Maybe even a stint as a scout/pirate? Throughout the game, you witness the after effects of an Avatar's destruction. In later levels, you actually see glimpses of the Avatar in action, perhaps being the key to victory in a major battle. The last act in the game, you are sacrificed and become the Avatar and you murder everything in your path for an entire level until the showdown with a greater demon.
Special guests would include Harlequins, dark eldar, farseers, and Phoenix Lords, all used to tell the Eldar's story and define it's unique racial identity in the 40k universe.
I want to play this game.