Or that they'll all still fall under the blanket of the "Imperium", but are simply factions within it. Like the Radical and Puritan factions of the Inquisition, for an example that already exists in the fluff. Maybe not a full civil war, but still a cause of friction within the Imperium.
Same thing could be said of the Tau, too. Some could be opposed to this union of sorts and would rather subjugate humans as part of the Tau Empire. Others (Farsight?) could welcome it.
I'm staring to warm up a little to this idea the more I think about it.




