Factions just released a week or two ago, so Mana should definitely focus on that before thinking about any new gamemodes. There are clearly a few bugs to be ironed out with it (as well as on other servers), and adding another mode would just add on to the list of issues of Mana.
Also, as Tas mentioned, given the current state of staffing on the server, we likely wouldn't have enough staff for the mode, as several current modes are, even, lacking.
Finally, there has been major lag issues recently, which I presume are a result of server issues, and a new gamemode with more players and more storage being used would just accentuate that problem.
If ManaCube does add minigames in the future, though, I'd suggest Skywars, as it's been on the server in the past, or some form of UHC.