Have the Western Empire's capital be in Gaul (as it was for a time in the 390s) instead of at Ravenna.
That way, any invasion across the Rhine is seen off. The Barbs may overrun Italy instead, but if so they are sandwiched between the "Gallic" Empire on one side and the Byzantine one on the other, and sooner or later get squashed. So the Empire gets a reprieve at least until the Arabs come along.
Of course, this may not be an unmixed blessing. It basically means that the history of the Western Empire is more akin to that of the Eastern, dying by inches over a millennium or so rather than collapsing quickly. If this leads to Roman culture being as thoroughly eradicated in Gaul and Italy as it was in the Balkans and Asia Minor, we've got a very different Europe.