What is the most useful potential feature which Emacs lacks?

[ndame]

There is a discussion on Reddit about sponsoring development of multithreading in Emacs, and people there say it's too hard, takes a lot of time and it doesn't even bring that much benefit to the user.

If this is the case (is it?) then what are those other features which could bring much more tangible benefits for the user and assuming somebody works on them full time sponsored by the community they can be implemented in, say,  a few months?