Tolkien himself considered revisiting Middle Earth for a new "uprising", but discarded it because it didn't and probably couldn't recapture the magic. Plus it probably felt too much like reality.
IIRC, he began writing another installment set in the 4th age, but abandoned it, because now that the main evils were dealt with, all that was left to happen was the world of men betraying each other, and it very quickly became a boring, depressing tale that just didn't need to exist.
And we can always add filler through nostalgia bait. For goodness sake, audiences still don't know where Aragorn got the boot he wore when kicking the helmet!