Due to the success of Nightmare Event (well, even moreso if it wasn't placed in the middle of mid-terms), I suggest of having a similar event, but this time with zombies! I stumble across the wikia of Left 4 Dead and found out they have a variety of zombies.

http://left4dead.wikia.com/wiki/The_Infected

So we don't not just have regular zombies, but mutated ones with specialized abilities.

Because they are LOL ZOMBIES and may or may not be much easier to kill than the other monsters last time, I would recommend of extremely lowering the powers of the most powerful of individuals while those with not as offensive powers will find them to be also extremely limited or removed entirely. For example, we take away Marco's ability to morph T-Rex, rhino, osprey, or any other morph that could help him easily escape or easily take over the zombies.

Of course, there is always the option of making everyone powerless and force them to take on zombies the odd fashion way. XDDD