The Riot chat app could really use some UI/UX improvements. It's a bit clunky and complicated. Riot is the most popular client that implements matrix protocol which allows people to chat/call and more regardless of the platform they are on. Matrix is similar to how email works (e.g. Hotmail users can talk to other users of hotmail, or users of a different provider), but with the modern features of instant messaging, typing notifications, and message-read indicators (and more). It also has the neat feature that you can create "bridges" to enable platforms that don't support matrix to send/receive stuff to people using matrix.
I think improving the Riot software would be really important because it helps people to communicate freely without being locked into a walled garden. If the UI/UX were improved more it would make it easier to encourage people to try it out.