As you know, this is the apocolyptic event which occurs at the end of LOTR. I've been searching non-stop but simply cannot find if any from say, War of the Ring will return. People like Gandalf, The Witch King. I know Sauron and Morgoth and the other Valar do, but It's more the likes of say Saruman. He specifically confuses me. Would they make use of Saruman? Would he be brought back? What confuses me about him though is, who would he serve? Morgoth or Manwe?
The majority of the work on the Second Prophecy of Mandos (which is the primary source for the events of Dagor Dagorath) dates from the period before LotR was written, so quite naturally it contains no references to characters from LotR returning. Asking about them would lead to this question being closed as "primarily opinion based".
We do know that the following are going to return:
The host of Ar-Pharazon who invaded Valinor in the Akallabeth:
they lie imprisoned in the Caves of Forgotten, until the Last Battle and the Day of Doom.
Turin Turambar in the prophecy text itself (HoME 5):
and on his left Turin Turambar, son of Hurin, coming from the halls of Mandos; and the black sword of Turin shall deal unto Morgoth his death and final end.
Although an alternate tradition (in the Athrabeth Finrod ah Andreth) has a slightly different role for Turin:
Turin in the Last Battle should return from the Dead, and before he left the Circles of the World for ever should challenge the Great Dragon of Morgoth, Ancalagon the Black, and deal him the death-stroke.
And that's it. There is further mention of events after the battle, so we have Dwarves:
their part shall be to serve Aule and to aid him in the re-making of Arda after the Last Battle.
And Men (Silmarillion):
Yet of old the Valar declared to the Elves in Valinor that Men shall join in the Second Music of the Ainur.
But nothing about the Elves...
Like I said at the outset, anything else is going to be "primarily opinion based" and would lead to this question being closed.
Saruman did not return because he breached the terms of his mission.