[Galorme]

I can give you a few things that will happen. The sun will crash into the world, and Morgoth shall decend. The men who were enchanted around the enchanted isles shall awake. The silmirils shall come forth from the sky, from the sea, and from the earth, and their light shall rekindle the trees. The elves will be old, and wise, and they will be very unlike they are now, being at last old. And the armies of the Valar will fight the armies of morgoth, and the Maia shall fight Maia, and man shall fight man, and the spirits that the elves have become shall fight the wraths and spirits of Morgoth. Morgoth will return as a beast of power. He shall be faced with Fionwe and Turin, and at the hands of Turin's black sword he shall perish. But the world will reach its end, and the lifespans of the elves also, and they will die, the death that they have long envied of men.

And all things on Arda shall perish, save the Valar and the Maia, and the souls of men shall be gathered in the halls of Eru, and there they shall make a great music. And each shall see in the song themselves, and how their actions changed the world, and made it better and furthered the beauty of Creation. And Eru will see that all he has created enhanced the glory of himself, and seeing that the new song is enhances, he shall give it being. And the world will be made anew.

Feel free to chalange me on any part of that, it is a mishmash of old legends, discarded stories, and combinations of storys based on assumptions. But I think it gives a pretty good account of what will happen.