So we could have Rome build a brand new city in the area North of Switzerland between the Danube and Rhine.
Thus you have the Alps covering the south approach, the Danube blocks off most direct east paths except for a few narrow passes, the Rhine does the same, to a lesser extent, for the west, and the gap in between the 2 can be fortified to high-hell.
Interesting concept, but you would be in South-Western Germany, in Southern Swabia, probably on the Northern shore of the Bodensee.
However, having a look at the map, it looks rather odd to have a major Roman center there - I would rather pick a place South of the Rhine, OTL's Konstanz oder Basel come to mind here.
Considering a Western alternative to Constantinople, I dismiss Ravenna for exactly its marshes and swampland. I would say there are limits as to how big the city could have grown.
Treverorum is not a bad place, being about halfway between the Alps and the North Sea while not directly on the Rhine. The best centre to react to Germanic threats against the West. However, it is set in a hilly region with little communication besides the river Moselle. Maybe a place further west would be able to have better communications for a more flexible response. I forgot the Roman names for Verdun, Reims and Metz, but these cities existed back then. I would place a permanent HQ in this region.
I wonder why nobody mentioned Mediolanum yet. It was capital of the Western Roman Empire for some time. In my opinion, though, it is on the wrong side on the Alps and simply a place to defend Italia.
People mentioned Londinium, then why not Lutetia? It is almost as good a position as the one I mentioned before, plus it sits on a possible defensive line on the river Seine. Also, the land is fertile and not too hilly. It makes sense that we see one of the most magnificient cities of OTL there.