I know that multiplayer has taken some people by surprise (it did for me), with nobody really expecting something that, with a little tuning, could take on L4D or Killing Floor.
But what do you think would make some good things to add? Game modes, exclusive weapons, single-player bonuses like suits and guns, etc.?
Here's one to get you started: a story mode!
Just think L4D, but with... well, alien zombies. That's basically it. I'd imagine it would be a bit more story-based rather than "HI GUISE, THERE'S ZOMBIES! LET'S GO KILL THEM AND GET ON A CONVENIENTLY PLACED HELICOPTER LOLZ!", perhaps even setting up stories for individual characters, and even putting character-specific sections, so if, say, there was an Aegis 7 survivor (because there's like, twenty of those, apparently), that person may occasionally slip into his/her dementia, forcing others to help him/her.
Or, we could pull an Alien Vs Predator and have multi-species death-matches!
Imagine it! We could have Necromorphs fighting on the side of the humans for no adequetly explained reason! Teams of Security officers using tamed and leashed Necromorphs to kill off wanted terrorists/EarthGov Resistance/[insert favourite Ethnic group here]!
I'm not too good at this kind of thing, so other to you, inteligent editors!