The Foederati idea of the Roman Empire generally had 2 outcomes - the Foederati romanised or they became an elite class above a Roman peasantry.
How possible is it to get something in between? Where there are 2 coexisting types of Imperial citizen - e.g. Roman and Goth - who have specific roles in the Empire and one is not held superior by bloodline.
Ideally a citizen could pass from from type to the other on meeting particular requirements, albeit not easily enough that the types merge, nor too difficult that apartheid sets in until one is overthrown.