Our German branch posted a recent interview with game designer Swann Martin Raget, and asked him about the lack of a co-operative mode and the possibility of a sequel.
Why is there no co-op in Zombi U:
"It was a bit more a question of the time. You can't do everything at first. We really wanted to have a compelling singleplayer experience and everything. We tried several things with co-op. They are here and they worked pretty well actually, so it could be a good idea. But it is not in the game, because for the moment we want to have the best singleplayer experience that you can have."
But as of right know there is no work being done on a possible sequel:
"At the moment, nothing is planned. We really want to see first, how the players react to the game. We have lots of, lots of ideas, that we would like to try."