Ending in FPS sounds VERY cool, hope they pull it off by not doing like that House of the dead movie where they put stuff from the video game in, it looked horrible and didn't fit in.
I'm looking forward to the main character, he seems like a cool guy and really fits the whole being a marine. What might be a problem is the other actors, that will be filling the positions for the other marines. I'm pretty sure they will have the black guy who is very funny, then the nervous guy, then the hardcore guy that is so extremely tough. Aliens have these people too, Predator had them and basically any other movie with a group of actors.
That the BFG isn't being called "Big Fucking Gun" I suspect have to do with the movie being rated PG-13 or so.
That the zombies or whatever will be a virus is somewhat annoying, in the games it's from hell or some weird dimension, but it's for sure not a virus.