Mission Control git master (allegedly going to be 5.3) has quite a lot of changes, and in practice distributions all use 5.3.2.
I suggest we branch at 5.3.2, cherry-pick only bugfixes, and call the result 5.4.0; meanwhile, when git master gets released (which I think should be sooner rather than later), we should call it 5.5.
It might be worth making an exception to the bugfix-only thing to get Bug #27309 (Observer.Recover) added in 5.4.0, since it's a smallish change and not very controversial.
For as long as we can (i.e. until they diverge too far to make this practical), we should make all future bugfixes against the telepathy-mission-control-5.4 branch and keep merging it into master, like the way telepathy-glib works.
The proposed stable branch:
Related changes for 5.5:
Fixed in (obviously) 5.4.0