MEGOLM seems to be encrypted group chat. OMEMO already provides that.

Cross signing OMEMO keys would be nice indeed.

Not sure what a session is or why it needs sync.

XMPP is decentralized, maybe even more than Matrix, since almost everyone uses the official server matrix.org.

I'm not sure if all of this is reason enough to switch to an non-standard protocol, fragmenting the small space of open IM protocols even further. It would be better to implement those features using existing Internet Standards.