mine is perhaps a mastodon with dislikes
a loops like but for youtube shorts instead of tiktok
email (i am not technical but since it is hard to self host email what if someone made a email that could be self hosted that is a part of the fediverse)
and a facebook like that isnt outdated as friendica
one that is similar to indeed forums/ glassdoor reviews which allows to segregate fields, jobs, or certain areas of the industry by category, but without the astroturfing and shutting down, the only problem is getting a huge userbase to use it.