The idea of Paul McCartney making music for a sci-fi FPS is pretty strange.
Not really. He's a fan of sci-fi, for one. He even got Azimov to write a sci-fi movie treatment for him once. And musically, he's actually VERY diverse, and his compositions quite good. He's not really the "doodoodoo...yeah, yeah, yeah" people discount him as at all.
Consider just one idea for a moment...Set this picture in your head:
The Hunter class has been described as influenced by the spirit of the Wild West bounty hunter/gunslinger. "Hunters were once outlaws who roamed the wastelands and wilderness making their own luck, and their heritage has led to their characteristic traits of bending the rules, combining daring and ingenuity. Hunters serve as trailblazers amongst Guardian classes, scouting hostile and dangerous areas."
Now picture yourself, as a lone Hunter, scouting a barren landscape, like a Sergio Leone film, and hearing this music as you step out to brave the unknown:
http://www.youtube.com/watch?feature=player_detailpage&v=vH5sucZ6FKI#t=11
I think that'd be pretty epic. (Thats from his score of the movie Honorary Consul, BTW)
...
Now consider also that in the same guy, you also get a dude who has made stuff as diverse as the Vanilla Sky Soundtrack, or can smash out something like "Live and Let Die" too...
And one should really consider the music for the 2011 ballet "Ocean's Kingdom." The ballet sucked and bombed, but the music was praised. I think you can get a good idea of some divergent movements in the four clips presented here:
http://www.youtube.com/watch?v=RzUeYT5LMy0
And hell, even if Bungie just hired him to come in and sing that one line From Venus and Mars/Rock Show -- "Venus and Mars are alright tonight..." and have that guitar hit, It's make for a pretty cool moment in the game.
Find me a song he's made that you think would be a good fit.
Has he ever composed music for games or film before?
The above should answer that question, but...Yes. Quite a few films actually. Including at least one nomination for an Oscar for the score of Vanilla Sky.