I know copyrights would be an issue, but imagine if we got a FPS game featuring various iconic characters of that genre. I was thinking it'd be mostly a deathmatch game that could be played offline and alone against bots, but a campaign mode, with a story and all, could work, imo.
As for the roster, the possibilies are vast. Doom guy, Duke Nukem, Lo Wang (from Shadow Warrior), Caleb (from the Blood games), Joanna Dark (who'd sadly be an X-Bone exclusive, if such a game was released on it), Master Chief (same as Joanna), Gordon Freeman, reps for Turok, Hexen and Redneck Rampage (a game I personally liked), Hidden (from The Hidden: Source), I could go on.
You could get a few non-VG characters as well, like the Alien, the Predator, the colonial marines (from Aliens), James Bond, and the Terminator, and VG characters from non-FPS genres.
Each characters should have weapons from their games that only they could use, like the cerebral bore for Turok characters. Throughout matches there'd be ammo for each contestants' weapons, but it should be possible to take other characters' ammo, to complicate things for them.
Personally, I'd like a few retro maps, that seem straight out of a FPS game from the 90s, and every characters should have a 90s model, a little like Mr Game & Watch. Both these designs and the modern ones should be selectable no matter which map you're playing on.
Again, I realize copyrights would be an issue, but never mind that. How would you people feel about such a game, if it could be done?