Read/unread and tags are all supported by IMAP already so that should be possible for sure. Same with starring/flagging email, any willing mail client could implement those.

The big integration points I see are within special features such as some kind of cross device PGP support, the ability to sync customised "from" addresses, and other features that the more basic built-in email apps lack.

Perhaps the code could be further extended to also support calendar, task, and chat functionality, just like on desktop, but that would take more effort.