I'm skeptical of starting a ZombieMod or Zombie Horde (if that still exists) server based on past experience, that experience being that (1) these mods have been fairly unreliable, flaky, and poorly-documented, and (2) even after being successfully installed, these mods proved to be fairly time-intensive to set up in terms of map selection and mod configuration. If you're familiar with ZM or ZH (if that still exists) and can identify for me a working (stable) plugin version, a set of maps to use, and a list of good values for key cvars I can definitely set up a server.
Although CSS Minigames and GunGame were only installed two months ago, unfortunately they do seem to be already completely forgotten by Zero and Allanon, and have been substantially unused. If anyone plays either of these games, I'd be happy to take suggestions for maps to add or reconfiguration to settings.
Key thoughts for starting a new server for game X:
It would be good if there are several members of the community (ideally some admins) who already play X several hours (each) per week or more.
It would be good if X is actually fun to play with a small number of players, or, alternately, if servers running X are so scarce that the server will fill up very quickly.