I don't think that's the issue, plenty are already. The issue to me is I'm not going to use something my friends/family aren't using. Maybe something matrix like where many clients are interoperable will work? I still think to take off it would need to support being a frontend for imessage/whatsapp/messenger too or no one will start using it, in a similar way to how imessage falls back to sms, this theoretical app could fall back to whatever shared app the two contacts have.
Matrix is overly complicated for the purpose and XMPP/OMEMO already are interoperable between many different clients. Just like all other communication systems it is the network effect which makes them usable. Tell your friends to install a client of choice and - for those so inclined - run a server or create an account on one. Keep your current W/app or Telegram or whatever active for now while you slowly move more communications to XMPP. Once you have contact with most of your friends and family via XMPP make it your default wat to communicatie, i.e. do not start conversations over the legacy apps and answer those who contact you over them through XMPP. You'll find that you'll end up using those legacy apps less and less. Keep them active if you want but don't initiate conversations over them and you'll be set for the moment using those services becomes untenable.
This is not just fiction, it is what I have done and am still in the process of doing, in my case moving from Telegram - I never used nor will I ever use things which requires accounts run by metafacebook or Google or Microsoft or any of the others.