No, there is no good proposal for good interop. And mosaic won't succeed unless someone invents such a mechanism. I tried to make the point here https://github.com/SteveFarroll/mosaic-spec/issues/6 but even that is a bad idea (dual stack). So yes you are right it would fragment the user base and so I'm not in favor of it. Because users matter more than software, and software matters more than formats/specs. And so no matter how good you make the formats/specs, without awesome software and without the power of the network effect, you get nothing.

But that doesn't mean mosaic isn't interesting or can't be useful for ideas. Nostr might be able to borrow ideas from mosaic, or attempt to approach it in some ways.

Or maybe someone smart figures out a brilliant way to phase over. Who knows.