I'm am new, but I've done a lot of reading of this board before I come out and spout out my possible rubbish. I may repeat a few theory's though, as it's impossible to read every thread and post.
1 - The Lighthouse, maybe the special rounds will be in the dark if the lighthouse lights the map. So, it will still be Zombies, (not monkeys or Zebra's), but normal Zombies, just in the dark.
The exciting news 3arc have been talking about
2 - Maybe the rolls reverse, perhaps the player becomes the Zombie hoard, in some sort of spun out tactical game. I hope not.
3 - The rounds become part of a level system. 5 round then a end of level boss/super Zombie. Extra health, perhaps a weapon, you decide, just an idea.
4 - More players, 6 players perhaps, this is doubtfull though, as the chracters have already been announced.
5 - Teleporters, these may take you back to older maps, as the older chracters.
Finally some sort of leveling system would be cool for Zombies as a sort of indicator as to how you've done and your progress, not just.......nothing.
Either way, just more of the same would be cool.