Will a "definitive" version of The Silmarillion ever be released?

[Legolas]

I don't know how much room there is right now for a conclusively more 'definitive' text because of the state of the stories left when JRR passed. Christopher Tolkien has commented on this extensively. He was pressed to complete his version quickly by the volume of requests, and of course, several writings were found after his 1977 version was published. Given much more time and all the material he had by the time the 12 HOME volumes were finished, he's said that he could've had a substantially different version.

He set out on the difficult task of trying to use his father's latest versions of the stories, but to also balance completeness and internal consistency. Few stories had all three qualities. Stories decades in the making are problematic as they change so much - in revision, later versions would change details that also affected the accuracy of/continuity from earlier stories that weren't updated yet. Doesn't help that JRR's handwritten notes often weren't legible!

Unfortunately, at his age (87) and the amount of time (20+ years) he has already spent editing and publishing all of these writings (The Silmarillion, UT, HOME), I suspect he'd rather spend these last years of his life on something other than a new Silmarillion, and mostly I can't blame him! The Children of Húrin and The Legend of Sigurd and Gudrún are testaments to this. (On the other, selfish hand, part of me says - what a crowning achievement that would be.)

If a new Silmarillion was to come from his hand, I think it would've arrived shortly after The Peoples of Middle-earth (HOME XII, 1996) in the late 1990s/early 2000s.

If not from Christopher Tolkien, I think it could be difficult for a new version of The Silmarillion to be attempted, published, and received as definitive. Not impossible, but improbable - it would have to be impeccable! I am largely ignorant of the amount of scholarly study given to Tolkien's work, but if it is there, maybe it could happen. Time may make it more likely; as JRR's children pass and the Estate is in the hands of another generation, I wonder how it will hold his legacy, including if/how it will continue 'new' releases to feed longtime fans and stir up attention for new ones.

In regards to "expanding" the tales, I think that's less likely because some of the more detailed versions of the stories were also earlier and/or never completed - thinking particularly of "Of Tuor and his Coming to Gondolin" from Unfinished Tales.

Originally Posted by Galadriel55

No, but he wanted to change some details. One that I remember is how Gandalf couldn't read the runes on the Elven blades they found in the Trolls' cave. TH says that Gandalf didn't know them. JRRT wanted to change that so that the runes would be covered in muck/rust/etc and he wasn't able to read them until he came to Rivendell; this way Gandalf the Knowledgeable that we see in LOTR is the same Gandalf in TH. I think he also wanted to change something with Gollum.

Tolkien commented more than once that he was having to resist the urge to rewrite The Hobbit for continuity of tone with The Lord of The Rings, and according to The History of the Hobbit (John Rateliff, 2007) he actually started a rewrite that was scrapped by chapter three because it felt too unfamiliar. Someone previewed it (I can't remember who) told him it "just wasn't The Hobbit."

He actually did change some details. Particularly the fifth chapter (the account of Bilbo beating Gollum at the riddle game) was revised for the second edition of The Hobbit in 1951, done so to reconcile the version of events in The Lord of the Rings. In the first edition, Gollum willingly put the Ring at stake in the game, and when Bilbo won, they parted ways peacefully.

Further changes were made for a third edition published in 1966, primarily because, or at least the opportunity was taken because, of copyright problems Tolkien was having in the US with Ace books (who were printing his works without permission by a loophole - that previous US editions only had UK copyright information listed). I do not know the specific changes made for the third edition; they might be included in The History of the Hobbit, but my copy is in storage at the moment. I expect that these were more philogic concerns.

There's a note explaining both revisions (1951 and 1966) in the front of most editions published since.

* TL;DR - Probably won't be another Silmarillion. Tolkien did revise The Hobbit twice, and started a rewrite that was scrapped early on.