Eg: chat, achievements etc.
The future of chat is being evaluated, so it's currently unknown whether it'll even be added to UCP or when it'll be added otherwise.
And the achievements feature is expected to be added later this year under an upgraded state.
Referring to the new Achievements as "upgrading" is likely to be inaccurate. From what we have heard so far, Fandom is trying to integrate it into Gamepedia's achievement system. As such, I expect that it will work noticeably different once it is "converted".
Edit:
I also want Chat to come back. However, I think Fandom is more likely to drop the feature than keep it.
Have you looked User:Noreplyz/UCP#Missing features?