Skallagrim
Banned
But they did allow assimilated barbarians to become Roman after multiple generations of having been conquered.
Yes. They were absorbed into the Roman state. They were to be loyal to Roman authority thereafter. If they become Roman collectively, in a group, their erstwhile leaders/chiefs/kings would regularly be integrated into the Roman elite somehow. Those leaders stopped being sovereign rulers.
And that is an option. I even mentioned it earlier:
And now we come to the latter days of the (Western) Roman Empire, and areas formerly held by Rome have fallen away. There are barbarian kingdoms there, whose ruling classes have often adopted Roman culture to a nice degree, but who are still barbarians. From the Roman perspective, these areas can of course re-join the empire (barbarians and all, one assumes), and renounce any claims to independent sovereignty. That shouldn't be a problem. It'll even turn a lot of barbarians into citizens. But there will be no independent kingdoms anymore. They'll just be a part of Rome again. This is unlikely to be something those barbarian kings would want. For starters, they'd have to stop being kings...
If Rome can get any or all rulers of the post-Roman states to agree to that kind of a deal, that's fine for Rome. But I doubt those rulers would go for it without very good incentives, as I said. More importantly: we'd then be looking at a resurgence of the Empire-that-was, rather than at the emergence of an alliance-based Empire of Trust. Personally, I think both options allow for very interesting avenues of exploration, but the OP specifically asks for the Empire of Trust.