I think you should try to implement a goal system where if you kill 100 humans as a zombie you get XP or level ups. The concept is just to try to get zombies to play more. There has been a problem with zombies afking and I think if you add this the game will be more enjoyable for everyone.
100 humans is a lot. The most I have ever gotten in one game as a zombie is 7 or 8 so far. I think giving a level to both the zombie with the highest number of kills and the last player alive is more fair.
I was just giving an example and just wanted to explain the concept and your idea sounds pretty solid aswell. you could do like 5 or 6 lvls for 100 human kills or 1000 human kills for like 20 lvls. idk something like that. Give zombies a long term goal so that they consistently play.
I think maybe you should just get a LOT more xp for killing humans or something. But the milestone thing or at least the idea of a reward could work, so Support
I support this massively. The amount of AFK zombies I see is ridiculous and players should be active more especially zombies.