[democrite]

Yes would be nice but of the requirements, there would need to be some common sync platform and participants. Any commercial source, like iBooks, Kindle, Kobo, or B&N Nook will likely stick to their own sync, for being able to verify sync reliability, maybe other issues, plus interest in selling books from them. But a common platform to store highlights and 3rd party apps syncing with it, could be possible but no one perhaps has attempted it.

Though there are also sync issues. On iOS, many will choose CloudKit since it is zero config. Another method of syncing, Parse, I think they closed and went open source so everyone has to have their own server. Yes, a common sync solution could be possible though it seems the needed min is a site and a way of syncing available on both iOS and Android that programmers could employ. Then again, there is also the question of how to determine what book is what so it's known it should be synced. For that, even sync of Marvin with Marvin is a difficult question.

Not sure if later it'd be possible to import iBooks annotations, maybe partially per book by text string matching, sure would be nice. But there are enough things to do that I don't mind if it's delayed to never.