After having play ZEM, I found some concepts very interesting, and i'd like to propose some of them to improve ZAM.
1) The variety of zombies : with return of the tank zombie ( which get like 3000 hp but is very very slow ), and the invisible zombie, which u can see but with more difficulty, or also the healter zombie which heals zombies around him.
2) Possibility to improve a weapon : you buy a gun in the shop class, then you can get improves for it, first improve is for exemple dual mag, then red dot etc.. ( something like 4 or 5 upgrades )
3) Which is in my opinion the most interesting concept : the gun&knife mod, which appears randonmly on some map on ZEM. In this mod, hunters aren't allow to put barricades and get some HP. So could be added in zam at the moment of the vote for restart or nextmap an option for this mod, that would make ZAM more diversified. Maybe for this option won't be abused it could not always appears in the vote.
The idea is not to copy ZEM, but just to get inspiration of what they created to make ZAM better.