A UHC would be really cool to have on CC. However, I don't think this is a realistic idea. Adding more gamemodes runs the risk of spreading the community too thin. The introduction of new gamemodes could reduce player-count in other games such as line dash, survival games, skywars mega, and eggwars solo - the open access games with the lowest concurrent players atm - meaning matchmaking could take a lot longer and some opponents could be in multiple consecutive matches.
What a new gamemode tries to accomplish is to bring in enough new players to offset reduction in concurrent player counts. We see this happening with skyblock. I personally think UHC will not result in this offset, not to mention that it will probably have a low playercount since UHCs are typically oriented to the more competitive part of the playerbase. UHCs will likely suffer from longer queue times as many players need to be in each game in addition to the generally low playercount.
I think UHC can still be a feature on CC. Making several modes - UHC, Duels, etc. - available to only single parties (as possibly a VIP perk), and not a public queue, will provide the opportunity for UHC to be played without long queue times and without the fear of being demolished by tryhards each game. I'm not sure if I articulated this well, but I hope you get the point.