The PL Rotation server is always empty, and it takes an event or rewards to get even a few people to play on it. I agree that degroot should be changed, it's a fun map that could be put on RTV or for events, but it is not worth a 24/7 server devoted to it. No need to start paying for a new server when we have empty ones sitting around.
If a 5 cp rotation is made, I'd also like to include warpath as well as badlands on the rotation in addition to the ones that Sequel Sentry stated, as they are both solid maps; warpath may be custom, but entire servers have been dedicated to it before and they do quite well.
If a valve maps (and convoy) is made, I would alter the rotation so that arena maps are only on the server at times when there are few people on, as going from say, dustbowl to lumberyard, with 24 players doesn't usually work out well, as many players often leave. I don't see why you includes so few valve cp maps on your list, Sequel Sentry, as there are a lot of other good maps out there to be had, if not in the rotation at least in the RTV list.
A funmaps server could work out really well or could fail miserably, I've seen a lot of both cases, and not much in between. It all depends on exactly what types of funmaps you put on, and how dedicated members would be to making sure its populated.
I don't know much about zombie mod, but it does sound like it's at least worth a shot.