I think we have to refer back to the poll and accompanying discussion that was had recently (Legacy maps [VOTE] Vol 2), as it frames and guides the discussion a bit better.
Mr. Muppet indicated a map limit on the server, that 'equates to 26 custom maps as per the ETPub mod'. Which maps are custom maps, or which maps are not custom maps? If I am assuming correctly, the core six are non customs: Radar, Oasis, Railgun, Goldrush, Fueldump and Battery. According to Charlie's list, 29 maps on currently on rotation. Which leaves us with 23 custom maps, and 3 open spots for new maps to be cycled in. As I understand it, pulling one of the core 6 maps out, does not open up room for another custom map to be added, which is relevant further down.
In that topic, people have already made some suggestions which maps could be removed (detracting those that have already been deleted since): Ice, Battery, Reactor, Missile, One version of Oasis, et_mor2 (also know as Marrakesh), Erdenberg_t1, Et_beach and Pirates (Recently bugged out on the server: The gold was delivered onto the truck, but the map did not end). To build further on this list, I think these could up for consideration as well: A version of Supply Depot, Venice, One version of Goldrush, ETL Bergen.
As said, a distinction between custom maps and non custom maps has to be made. If a version of Goldrush and/or Oasis is deleted, it might or might not open up space for a new map. This is not necessarily an issue, just a consideration. And the question poses itself with replacing Fueldump with Fueldump-Z.
The same topic also gives further input on which maps to add. Both Vesuvius and Colosseum got 5 votes, similar to the recently added Axis lab and School. Cortex and Village were runner-ups with 4 votes each. It would be unfortunate to throw out that input.
Deleting Goldrush and Supply Depot sounds rash to me. I do feel the same regarding their repetitiveness, and I have seen other people express a similar sentiment. Be that as it may, the fact that these maps are so often voted in does mean that a considerable share of the player base loves playing them that often. That should not be neglected.
There must be other means to promote a more diverse map cycle. As suggested, taking out one version of those maps might reduce their frequency. Or, given that it is possible, filter out maps that have been played recently for X amount of time. Alternatively, opt for an exhaustive cycle where all maps have to be played before the full range of maps is available again. That might already be too strong of a measure, and I have no idea what the impact will be on the amount of players coming to the Team Muppet Legacy server. I would completely understand and expect some hesitancy from the Muppet team too in this regard, as the server is clearly flourishing with the way things are run.