I prefer the idea of barbs being enabled to occupy cities. After some time (perhaps 5 - 10 turns) having hold said city, they could adopt the culture and form a civilisation of their own. That way it has been all the way in history.
The conquerors came, the conquerors won, the conquerors stayed, the conquerors were assimilated but built some kind of new society this way.
Additionally, this would make barbs become a much higher threat than they are today. I mean, what happens if they take one of your towns? You loose the production, you loose 1 pop, you loose some money. But after that, they are gone and you can proceed. Annoying for sure, but not really devastating, as barbs have been in history.