A proper 40k linear fps would be brilliant.
The best two options I can think of are;
1. A kick arse 7 foot tall hate filled bio-engineered brainwashed prayer swilling ego-less killing machine in a powered exo-skeleton covered in armour (space marine), who as the last survivor of an expedition (through no fault of his own) goes on unrepentant and without conscience in a genocidal rampage, watched with appropriate fear and horror by his human would be allies, who can acheive little more than pity for his enemies.
2. A fearful and underequiped generic soldier (guardsman) who's superiors not only will kill him for not following his orders, but will kill him for asking questions, for not going beyond the call of duty, will kill him for requesting ammunition in the wrong colour ink etc. And have assigned commissars to each platoon to spy on the soldiers and personally enforce these punishments, shooting the soldiers in the head with very little warning. The main aim of the game is to increase in rank, primarily because with each promotion the commissars and superiors have to talk to you for longer before killing you, thus giving you a chance to make excuses, run, or fight back. Killing a commissar when there's no officer around will actually go down well with the rest of the troops, but his replacement is bound to be suspicious.
Going up in the ranks involves surviving battles with various alien races, none of which you are properly briefed on, and some of which superiors will disbelieve the existence of even after you rescue them from them. Certainly claiming that these aliens are the equal or better in combat than humans is an offence punishable by death.
I've gone on a bit about option 2, but I really want to play this game, and I bet I'm not alone.