Yeah, you'd have to migrate an entire population to effectively be able to recruit armies from a place where none of your people have lived before...

And about reemergence... it depends on what regions you would count as core... for teh Romani for example, if Rome would fall to, say, Gaulish hordes and a few decades down the road the Samnites revolted against the barbarian overlords it's highly unlikely they would reinstute Rome after winning their independence from the Gauls.