A Western Rome with peak borders. Would Rome focus on reclaiming the East, stay within it's borders, eiliminare and possibly conquer the surrounding tribes, or try attempts at colonization?
You can mold the Rome in any form you like, as long as there are minimal border changes to the empire and they identify themselves as Roman
Where would strong WRE focus?
I don't know, it depends on the current situation; it is not
that determined I mean.
If the ERE is weak that would attract the WRE's attention as it is the richest neighbor with the Roman population easy to incorporate, so that's the most natural way of expansion.
But if the ERE is reasonably strong, then the Germanic territories and the Berber lands are the objects of the Roman expansion and aggression.
That would be fortunate if the WRE is not too over-strained with fighting too many wars when the Great Plague hits.
It is
very hard to imagine a strong ATL WRE - you need a couple of "good emperors" reorganizing army the way that the 'ethnic' Romans are represented there substantially like in the ERE; and the Roman 'fat cats' are forced to pay their taxes like they did in the East or/and brutally repressed.
It is double hard to imagine the strong ATL WRE to concentrate on keeping what it has and focusing on the defensive measures: constructing the city walls, fortresses in the strategic places and other lines of defense, though it is it's best chance to survive the "Justinian Plague".
But 'strong WRE' is in more favorable position than the ERE - the Sassanid Persia and the Arabs are far away.