In terms of who “can” marry who, anyone can marry anyone. The issue comes with having children. The further apart the couple’s mana capacities are, the less likely they are to ever conveive, but if they both wnat to marry anyway (and their families or Aubs aren’t stopping them), they can get married. So Rozemyne is free to marry any noble, down to the lowest laynoble, if she wanted to, but to have any chance at all to have children, it’d have to be at least an archnoble. And to have realistic or even good chances to conveive, it’d probably come down to archduke candidates from greater duchies, royalty and Ferdinand. Or maybe someone from Lanzenave’s royal family maybe, but I don’t see that happening.

Within the royal family, we can pretty much rule out Trauerqual (since he already has 3 wives), Anastasius (since he won’t marry anyone other than Eglantine I’m sure) and Sigiswald (due to the pendant falling apart so easily). But Hildebrand would be a theoratical candidate, since with him starting mana compression unusually early by Yurgenschmidt standards, he might be able to get within a good range of Rozemyne, by the time he finishes his growth phase, if he works hard on his compression the entire time. And he has a massive crush on Rozemyne, which could probably give him the necessary motivation to do exactly that.