for me the most critical ones are replacements for discord and microsoft teams. for discord the critical piece is the login - people don't want to make accounts on each server, so until we have proper federation with a good user experience people won't actually move off it.

for teams i'm sure theres projects in development, i just don't know them or their status - all i know is that i want a project to combine several specialized FOSS services (jitsi is great, and there's lots of other collaboration tools for email/calendar/chat) into one nice unified frontend that is actually reasonably easy to self-host and maintain.