I think I'll go with the flag area defending. It doesn't require many players and allows non-competitive, more casual gameplay.
Some ideas regarding the AI, feel free to suggest more:
Base AI behavior:
The zombies seeks the closest flag and runs towards it. If the movement stops (Pos.x is close to the Pos.x a while ago) while the zombie want to advance, the zombie jumps. Each zombie has individual reaction time.
I might add some AI behavior game tiles, e.g. force jump & force move -tiles, so zombies knows when not to run directly in to death tiles.
Runner:
Runs directly towards the flag area ignoring the defending tees.
Shooter:
Finds defenders inside given radius (100 - 800 depending on weapon) and starts shooting / hammering if defenders are found.
Grabber:
Hooks defenders and tries to drag them farther from the flag area.
Freedom fighter:
Seeks the defenders and explodes when close enough.
Ninja:
Attacks defenders on sight but has limited vision (e.g. ninjas won't attack you if you're standing behind them).
Another variation could be predefined move & attack patterns, e.g. jump, slash, airjump, slash backwards.
Zombie lord:
Spawns weak zombies next to him if empty slots available.