The return of Sauron

[dancing spawn of ungoliant]

So what may be the new evils that could arise?

Have you read Histories of Middle-Earth? In Volume 12, I think, there is a kind of a sequel to LotR called The New Shadow. It is set in the 4th age and it tells about some evil men and orc admirers. Anyway, Tolkien ended up abandoning the idea of making a book about a new evil of which I'm quite glad.

Here's what Tolkien said about the story: "I did begin a story placed about 100 years after the downfall of Sauron, but it proved both sinister and depressing. Since we are dealing with Men it is inevitable that we should be concerned with the most regrettable feature of their nature: their quick satiety with good. So that the people of Gondor in times of peace and justice and prosperity would become discontented and restless - while the dynasts descended from Aragorn would become just kings and governors - like Denethor or worse. I found that even so there was an outcrop of revolutionary plots, about a centre of secret Satanistic religion; while Gondorian boys were playing at being Orcs and going around doing damage. I could have written a 'thriller' about the plot and its discovery and overthrow - but it would be just that. Not worth doing."

You can read The New Shadow here.