Realistically, what could the members of the accord do to Rhomania if they do annex/vassalise up to the Alps?
The Spanish are already in a perpetual conflict with them out East. The Romans will lay waste to any enemy fleets in the Mediterranean. Invading Italy via the Alps is logistically unfeasible, and there will more than enough men to repel any invasion. Rhomania is Arles’ largest trading partner, they won’t remain in the conflict long without massive subsidies.
They can ally with the Triunes and march endless armies into Imperial holdings in Italy, while flooding the Mediterranean with Triune & Spanish & Arletian fleets. If the Empire gets that huge the Accord will make common cause with King's Landing to contain it.
Yeah, as much I want to see a Roman Italy, until the Triunes a rolled back, or the Romans can build a counter-alliance outside of Arles and the Accord, Italy needs to be handled carefully. Sadly.
Although it does suggest that Arles and the Romans are going to drift apart, which does create an interesting potential for a war where Arles is Despotated alongside Italy in the distant future if the Romans do go further west. Though outside of Spain I can't see the benefit besides controlling a land route.
Tbh, if there is going to be Roman expansionism post-war then I'd expect it to be Mesopotamia/Persia (I mean, they are Timurids), Arabia or North Africa, MAYBE something out in the Far East. I think having Africa be Roman again would be very useful, especially if the Romans and the Accord fall out because a secured Africa can be one heck of a place to control entry to the Med from, with Tangiers, Ceuta, etc. It isn't Gibraltar to be sure, but still pretty effective. Which means the Romans can in theory defeat a divided Triune-Accord fleet.
It is weird to see potentially three diplomatic strategies the Romans could take at the moment. In-Accord, Pro-Accord, and Anti-Accord, all of which have some serious and interesting considerations.