[Azaelia of Willowbottom]

I think it's reasonably accurate to say that many Rohirric families were related to Gondorian families as well...but the question is, how closely? If you think about it, we could be all related (however distantly) to every other human on earth.

It's not much of a stretch to consider that your average Rohir has a Gondorian aunt, uncle, cousin, or grandparent somewhere very far up her family tree.

I don't think that a Gondorian would be elegible to be ruler of Rohan just because in order to establish royalty, one would have to search very far back on the family tree...much farther back than necessary to pick a Rohirric king. Also, would Rohan readily accept a king from Gondor? They value their independence too much and are too suspicious, I think, to just accept some upstart from Gondor as their king. This hypothetical Gondorian would have had to leave Gondor for good and live with the Rohirrim for many years to even be considered.

I don't know...this is all hypothetical, anyhow.