Though in all honesty, you can probably discount those three as being 'Imperial tribes' by virtue of the fact that none of them actually joined the Empire.
* The Bretonni explicitly refused to join with Sigmar's army at Black Fire Pass and migrated further west beyond the Grey Mountains founding their own kingdom.
* The Roppsmenn, as of newer fluff, were run into extinction by Sigmar for helping the Norsii invade Udoses land and raze Salzenhus to the ground and kill Wolfilla.
* The Norsii came down from the Northern Wastes and worshiped the Chaos Gods, becoming the ancestors of the modern Norscans.
Some material, like Tome of Salvation and the map of the ancient Empire makes reference to the Frikings tribe, but they were wiped out by Sigmar when they refused to join the Empire.