[SarumanCymraeg]

I can't remember the exact quote but I'm 99% certain that Tolkien stated that although Men [Men specifically, of course, the other races are all doomed to end, sadly] would face many new evils in the Fourth Age and beyond, they would never again face it in a metphysical, god-like, magical form. So no return for Sauron or Morgoth - or even the Blue Wizards. To that extent, Saruman, the WK and the Nazgul would not return either, and all the balrogs were gone. Also, Aragorn subdued the Haradrim and made peace with them, so they're not a threat.

So what do we have in Fourth Age Middle-Earth? Well, there ARE some evil Dwarves in the very east, but they're not going to be the new evil. Durin's Folk are declining*. There are also possibly dragons in the far north, so that's another possibility. The Barrow-wights in the north haven't disappeared overnight, either, and they might somehow possess someone to do great evil. To that extent, the faded Elves would also become wight-like, and the bitterest of them could well cause some mischief (not all Elves are as pure and generally 'good' as they seem to be portrayed]**.

Unfortunately, with Aragorn's death I think the whole saga and magic (somewhat literally) disappeared from Middle-earth, and the new evil would come from the hearts of Men.



*This is only a possibility. Although they sometime reconquer Khazad-Dum and were there 'until the world grew old and the Dwarves failed', that could easily be interpreted as centuries, or ages away, or even at the same time that Men eventually might fail.

**As we know the Elves who stayed in Middle-earth faded, becoming in effect spirits, although personally I feel that would be a long process, and that some physical Elves, in places like Eryn Lasgalen, would stilll be found many, many years after Aragorn's death. Therefore, any possession that they might do would probably happen in a distant future.