One problem I have with ZM is how even on the highest difficulty map, its too easy. Its too easy for the first 3-4 days then actually starts to pick up in difficulty and get really fun at the very end.
It would be amazing to have an endless mode gamemode/map where you play on day ∞, a wave based day that lasts forever and gets progressively more difficult as you kill more zombies. You can access the upgrade menu between waves and the time in between waves gets shorter and shorter until its one continuous flood of zombies.
This would encourage players to actually play with other people instead of soloing Compound (since its the perfect mix between unit gain and not getting bored) so they can not get slaughtered, get a high score, and more EXP once milestones drop.
EDIT: UnlisteadSky2 makes a good point about units so thats added to my suggestion I guess.