The independent people are great but i think they can have an extra role instead of only assimilation purpose.

There can be trait like the horde which will result in heavy production of military which will make outpost, attack player, capture city.

They can negotiate us and vassalize us or ally (but can't assimilate them) with them and at last there can be onset of great dissolution which will result in disintegration of empire.

There can be also be trait like pirating, the independent people can have outpost or a city the units will loot trade routes, attack other ships, vandalize resources.