[Mithalwen]

I don't think it is inconceivable though perhaps unlikely. The area makes better sense as two friendly countries rather than as a superstate given the white mountains between them. A lot would depend on population development and climate and also perhaps how long the direct lines of descent for the ruling families remained intact given the degree of intermarriage at the beginning of the Forth Age. In the real world, should Eomer's line fail maybe a Prince of Ithilien might claim the throne of Rohan (ok the Rohirrim seem to be less hung up on blood claims than the Gondorians but not impossible. An ambitious ruler of both Ithilien and the Mark might upset the power balance if a King of Gondor left a heir still a minor. It might be against the spirit of the victory over Sauron but it would not untypical of the race of men as we know them.