So after playing the game for the first time tonight, I'm feeling pretty hyped, and thus an idea came to mind. Since I'm new, this may have been suggested before, but hopefully not!
Anyways. The idea is pretty simple. What about making it so in later waves, or certain 'modes', when a player is killed a zombie is spawned that looks just like them and uses the weapon they had equipped when they died; thus making a mini boss out of all your fallen comrades?
I noticed the player playing as a zombie was suggested, and I agree that would not be a good addition, but for this suggestion the player would have no control; it'd be just another enemy that's even tougher to kill what with the weapon and all.
There are lots of different ways to implement this idea, too, so I'll leave picking the way up to you, and hopefully this proves to be worth using somehow.