As much as I would like to see some public effic ctf, I think it would not be playable with 26 players (this amount is reached frequently during peak hours), no matter what the map is. I think some servers also had mode-battles some time ago, just like you suggested!
I think we could try that out for effic: it's more difficult to form a voting majority with a full server (26 players), so that it's unlikely to happen, and it's easier to change the mode when there's a lower player amount (rarely happens on pastaland though).
As for the other modes, sure it is possible to allow other game-modes like hold, which I personally see as a more "popular" and playable flag-based mode after effic ctf, and in my opinion more game modes would simply require another server.
But we should not forget that as much as we can discuss those ideas here, as much do they have to be scripted, tested and implemented in order for them to work, which involves much work and time and volunteering.
So before we can hope that people find the time to work on it, many more people should be in favor of the idea first!