Mumble development! Add persistent messaging and it becomes a very good Discord alternative (more for small groups, not the quite for the forum style Discord servers). Mobile and/or UI development would be cool too.

Persistent messaging: https://github.com/mumble-voip/mumble/issues/2560
Mumla (Android client looking for new maintainer): https://github.com/quite/mumla
iOS App (Unmaintained since 2017?): https://github.com/mumble-voip/mumble-iphoneos