The Witch King....shall rise again!?!

[zxcvbn]

Originally Posted by Legate of Amon Lanc

First, Tolkien says that the Fourth Age and the ages later are only in the dominion of Men and that there will no longer be "mythic", "magical", or "supernatural", or how should I call that, enemies for them (I believe it is somewhere in the Letters - if anyone can provide the quotation, it will be helpful).

Yes, he said something about Sauron being 'the last physical incarnation of evil'.
But Tolkien kept changing his mind on many things. That's why he wrote 'the New Shadow' which featured a new Dark Lord rising just 100 years after LOTR. So we should atleast consider it a possibility.

Originally Posted by Legate of Amon Lanc

Another thing speaking against the return of WK without any Sauron to be his superior is the fact that as a Nazgul, he was totally subdued to the One Ring and when it was destroyed, his own Ring lost all its power, and therefore, his spirit departed, having no longer a mortal body nor any will binding him to this world.

The books say that the Nazgul were bound to Sauron's will, and not specifically to their Rings. Their Rings had already accomplished their taks(turning them into wraiths), so Sauron usually wore them on his hands rather than giving it to them.
If they were destroyed it isn't necessary that the NAzgul were also utterly destroyed.
As for free will, the Nazgul had it. Remember all the works of evil they did in Sauron's abscence?(Angmar, the Great Plague, Minas Morgul, running things in Mordor, sending emissaries to stir up trouble along Gondor's borders etc.)