The capital could become Constantinople and let's just assume the Empire will fall and consume the East for the sake of a good TL, OK! We need to make it plausible , don't worry.
Plausibility is tough. The ERE was by a long way richer than all of Europe put together: indeed, I'd estimate that Egypt alone was probably considerably wealthier than
all of non-Roman Europe in 400. Constantinople has a superb defensive location, too, but even should Constantinople be captured in some fluke before the Land Walls are put up, the barbarians have to deal with the rather obvious problem of the lack of naval supremacy that they hold.
If the ERE suffers a fifty year run of bad luck like the OTL West, then I suppose it's not impossible: but I just have my doubts!