If that's the case, then ironically a United Europe in 1400s (surviving Romans, an implausible line of personal unions with primogeniture, whatever your POD) might not have gone anywhere.
It's pretty much likely that with a PoD in Late Antiquity, Colombus wouldn't exist, and Europe could have gone in any (or lack thereof) direction.
That said, I think that Romania was going to went trough a domanial-mercantile economy (altough very slowly), delayed by the crisis. Mediterranean civilization, for what matter exchanges, arguably still mostlmy went trough what Fossaert called a tributary-merchant (which you could consider a particularily develloped ancient state apparatus dominating exchange economy). But the importance of the domanial production, which tended to the autonomisation and régionalisation from the IIIrd century onwards, was essential.
It's hard to really tell with a Late Antiquity PoD, how exactly and when exactly the transition would happen, towards a domanial-mercantile economy, where a financial and structuring power autonomous from the state apparatus, would direct at least part of the production. Abassids were on the verge of undergoing it, but it was crippled IMO trough successive crisis (altough
@John7755 يوحنا could speak more easily about this, especially in the light of the Revolt of the Zanj, a topic on which he's particularily versed) : with the preservation of Romania as a large Mediterranean political/economical continuum, I don't see why Romania couldn't at least be on a similar situation.
Long story short, there's largely a room for a mercantile development from Late Romania situation, especially in a situation where Atlantic/North Sea trade from one hand, and Danube/Baltic trade from another would likely devellop out of regionalized economies. Again, how and when, that's something really hard or impossible to state for me. But I don't see a good reason why it wouldn't, and some why it would.
As for an United Europe in the XVth, trough personnal unions...@Alex Zetsu, meet Hundred Years War. Hundred Years War, meet
@Alex Zetsu.
EDIT : For what matter Colombus, it might be amusing to think of him as "Jerkass Homer" (for the I'm always right and far too lucky for my own sake") Portugal as Seymour Skinner, ("Please stop speaking to us") and Castille as Mayor Quimby ("Well, it's stupid and I'll never do this. I'll do it, tough, because I can't say no to a potential big pile of cash").
It doesn't help this much, but it's entretaining.