Well, we know that Aragorn and Éomer dealt with any such threats:
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And wherever King Elessar went with war King Éomer went with him; and beyond the Sea of Rhûn and on the far fields of the South the thunder of the cavalry of the Mark was heard, and the White Horse upon Green flew in many winds until Éomer grew old.
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Anyway, I find Selmo's idea much better. As Tolkien himself probably realised as he wrote A New Shadow there were no more really great enemies left. All had been crushed in the War of the Ring and any story that would come afterwards wuld be nothing compared to what had happened in the ages before.
Maybe also rising tensions between Rohan and Gondor and also eventually the beginning of an industrial revolution, like the changes made by Saruman in Isengard and the Shire.
Who knows what would happen?