It would be nice if there's at least some strategic element for survivor mode. Would love it to be set in somewhat of an open playing field and the players can choose which routes to take, which doors to weld shut, where to place sentries, etc. Make it a real team emphasized mission to get out alive.
I think going for a "whoever lasts longest" type of deal, or something like L4D where it's get from A to B and cope with the swarms along the way, will all get very boring quickly. Plus, the mutants will probably just make it irritating, not challenging, IMO.