There is an exceeding amount of hostility on this forum for an Europe-wide successful enlarged Roman Empire, with people crawling out of the woodwork to pile upon bogus difficulties, such as Roman sailors getting shy of carrying supplies through seas they navigated for centuries. I guess it is because they want to get their Balkanized map of Europe.
I would very much want to see a united Europe, Roman or else. Well, unless it's a Nazi/Stalinist Europe, but this is wishful thinking IMHO. Sure the Romans could supply their armies on the Vistula, but it
would be a lot more costly. For a nearly useless province, they are sending their men into a wilderness, with so low production that even more supplies than OTL need to be sent from the Med, while the route is way more dangerous. And you're making Rome even bigger, which was one of it's weaknesses - when there was a mutiny/uprising, what followed was not suppression of those peoples, but reconquest of lost land. East of the Vistula there are still going to be barbarians. And same thing that happened with the Germanic tribes will happen to whoever lives there, Slavs, Finns, or some sort of Scythians. Maybe Goths. Over time they will develop, and from annoying natives they will become dangerous barbarian kingdoms. Against a frontier even further east, with all those supply difficulties. Like I said, I'd love to see a united Europe, but I don't think Rome conquering Germania is the way, not to mention what IMP CAES AVG said. He should know.
Oh and about the TL, how about "Rome conquers Germania, is even more overstretched, in 3rd century Sassanids, or *Sassanids come and it collapses"? That could be somewhat original at least.