A personal LOTR head canon-Tolkien apparently or at least according to his son abandoned Dagor Dagorath-to me it's still canon-it's satisfying that Turin would slay Morgoth and everyone comes back for the last battle with at the end Feanor repenting and the world being remade.
I don't like the Dagor Dagorath idea at all.
Turin is no good guy despite being a great hero who fought against the bad guys. Turin has committed too many crimes to be considered good enough to lead the Forces of Good against the Forces of Evil. I remember reading somewhere that Turin and Nienor would be cleansed of their sin of incest for the Dagor Dagorath and I was shocked by that primitive idea. Ethically, Turin and Nienor didn't commit the sin of incest because they didn't know they were siblings.
Nienor did commit the sins and crimes of suicide - and murder when she couldn't wait for her child to be born before killing herself. She acted as if her unborn baby was not already a person with rights, or as if that child was somehow accursed because of the accidental incest of the parents and the child must be executed for an accident committed by other people.
Turin did commit the sins and crimes of suicide - and murder when he killed Brandir because of Brandir's somewhat insulting comments. In the end, even Turin's sword, not noted for having high ethical standards, said it would gladly kill Turin because of the slayings of Beleg and Brandir.
I think the idea of Turin coming back and leading the Forces of Good in the Dagor Dagorath became obsolete once Tolkien created nobler human heroes, like Elendil and Aragorn, for example.
And what about the idea that Morgoth would be killed permanently?
Thee is a line in the English language Catholic mass "speak but the word, and my soul shall be healed".
Eru is supposed to be all powerful. Which means that Eru can merely "think but the thought of command, and Morgoth's soul shall be healed". So if a god is all powerful, what does it say for that god's ethics if the god prefers for an evil person to die permanently, instead of curing that person's evil with a single thought?
If Eru is all powerful and all knowing, the evil in the world proves that Eru is not all good.
Eru created a flawed world with flawed people in it who could turn evil, because Eru is like a writer who wants to write an interesting and exciting story with suspenseful conflict instead of a happy story like "once upon a time, the world was created, and everyone lived happily ever after". Write it with the lives of living beings. So basically Eru is evil enough to create a world full of evil just for fun.