I'd call this a pretty tall order. Somehow, what you've got to do is keep the Germanic tribes from migrating into the West and dismantling the whole thing.
However, I don't think it's entirely impossible. One thing you can do, ironically, is prevent Attila the Hun's death. He and the Huns largely kept the Ostrogoths, Gepids, and other Germanic tribes that later dismantled what was left of the West from doing so. If the Huns focused more of their energy on the East, which is something they'd done plenty of times before, you can save the West.
The scenario might go like this:
Attila lives long enough to name an heir/ heirs. Let's say 20 more years of his life has resulted in the mass destruction of the ERE frontier, while also bolstering the WRE's alliance with the Visigoths and has given Aetius a longer life as well.
The ERE is greatly weakened and loses a lot of ground in the East to the Persians after much of their manpower is spent fighting the Huns in the North. By 520 they have fallen apart, and all of their Asian provinces, and Egypt, are under Persian rule while tribes pour into the Balkans, repopulating much of the land emptied by the Huns. South Germanic might come to be an analogue to South Slavic in the region.
Now, at this point the WRE has already lost Britain, Africa, and Hispania. They essentially have held onto Illyria, parts of Gaul, and Italia. They might come to lose Gaul in coming decades, but manage to keep their enemies beyond the Alps.