I play Killzone 2 , i like a lot it's futurist look
COD4 it's realistic looking , but i fond it's too much near reality,
and Killzone 2 , makes you travel to some apocaliptyc world with futurist weapons, characters etc ...
That's fresh and good.

Caus all realistic FPS become really boring sometimes, they lack imagination.
Painkiller or Serious Sam 2 are example of non really realistic games with lot of fun !
What COD4 have over others is it's great class system in multiplayer, killzone 2 also have a cool system class even more pronounced and different from COD4 !

So what i could says , try to make an FPS with some originality , you can't surpass games like COD4 made by 200 professionnal people !

So yeah , make it futurist and bring originality if possible
Why not RPG elements or a Class system even if it is in single player mode !