I'm thinking no discovery of topics or people. No hashtag support. None of that shit. Basically just email-over-activitypub. You give it addresses for people you want to talk to and then it happens. Just like with phone numbers and SMS/MMS. Make it e2e encrypted if you like, or not.

The difference from email is I could have my nice iMessage like experience over here, and you can be stuck in Mastodon DM user experience hell on your end if you so choose. Choice!