XMPP

Stability, good user interface. While I am technical and enjoy playing with bleeding edge stuff, it is a struggle to get "normies" to join yet another chat system if they aren't as easy to use or reliable as signal/whatsapp. audio/video calls are also critical for most people I expect, if you want people to use it as a proper alternative.

My other half and I use monocles (used conversations originally, but had some slight annoyances). Both have a very nice interface, and are usually reliable. Now and again one of us doesn't receive the others' message, and I have no idea why, and it is it difficult to diagnose.

I only recently got my XMPP server to work through NAT properly for A/V calls, which took me ages to sort out, and the errors in the monocles interface were not helpful in diagnosing the problem.

Sometimes (rarely) monocles doesn't reconnect properly and one of us doesn't realise for a while, and then we get a flurry of messages once we reconnect. An easy way of telling when it's not connected would be good - the little icon monocles has isn't that obvious (esp now that android only allows white notification icons, which is really lame). Other chat systems seem to be more reliable in this regard.

I'm not sure if any of this helps, but best wishes with your project!