Several years ago, there was a thread on the Hero Games forums discussing ways to make a game set in the Fourth Age of Middle Earth work. It was agreed that there would be no elves or dwarves, or at least there would be next to none. Someone pointed out that in order to feel like Middle Earth, you needed a big bad guy...after all, in the Silmarillion, you had Melkor/Morgoth causing trouble in the First Age, and Sauron filling the role in the Second and Third Ages. There were some suggestions that maybe Saruman or some other corrupted Maia would be the bad guy; it was pretty generally accepted that the Fourth Age would have a lesser bad guy in keeping with the lesser grandeur of the age. After all, even Sauron wasn't as powerful in the Third Age as he was in the Second, and he didn't hold a candle to Morgoth even then. But the final demise of Sauron and Saruman was too well-documented in the source material. The thread wound down around that point, without really finishing what it had started.
The naysayers won that one, and no suitable enemy was to be found. I think Andres Diplotti figured it all out, though: