In The Two Towers, Book III, there's an intriguing conversation that happens between Gimli and Legolas. Gimli is waxing poetic about the caverns and halls of Helm's Deep, and in response Legolas says:
You move me, Gimli. I have never heard you speak like this before. Almost you make me regret that I have not seen these caves. Come! Let us make this bargain — if we both return safe out of the perils that await us, we will journey for a while together. You shall visit Fangorn with me, and then I will come with you to see Helm's Deep.
Yes, there's an alternate reality where Tolkien published a series of books in which Gimli and Legolas from the best-selling Lord of the Rings novels travel Middle Earth together, delving into the forgotten dungeons of the recently fallen Sauron, and the forgotten halls of the dwarves, and the still-healing forests of the land, finding adventure and lore every step of the way.
Obviously that possible world did not surface into our reality. There are no books about the Legolas and Gimli delving into dungeons together. As I've said many times before, Tolkien suffered the injustice of being the founder of a subgenre. You see hints of his world and lore in all modern fantasy, and it seems that most modern fantasy authors are pretty happy to acknowledge that. It's no secret. Many, if not most, fantasy nerds live and breath Tolkien.
The most uncomfortably obvious proof of this is the original version of D&D, which featured the literal race of "hobbit" until the Tolkien estate issued a copyright claim on the word. Maybe I'm reading too much into it, but I think the tight hold the Tolkien estate has on the lore of Middle Earth is one reason we never got posthumous spin-off series.
It's easy to argue that limiting Middle Earth to exactly one author, who also happens to be dead, has done a lot to "protect" the branding of the franchise (yes, I'm painfully aware of how modern a term "franchise" is when describing a book series). There have been several properties that have been let down by their expanded universes. I shudder to think of how much bad Star Wars fiction I've read over the years.
Then again, as David Collins-Rivera, author of the Stardrifter series, has said, an official canon for something that's fictional is nearly a literal paradox. It's imaginary. If you don't like a book that comes out, just don't accept it as true. Call it a legend, or a lie, or a mistake. You can build your own fictional universe.
We geeks are experts at this, in fact. We invented the multiverse, at least the pretend one that pop culture uses today to justify variations in storylines and actors and all the franchise reboots. Geeks knew the concept of an acknowledged multiverse ages ago, through comics and book series and roleplaying games. Heck, every time you play a published Dnd module, you're acknowledging a fictional multiverse. You're playing through a puzzle that's already been solved by other players all over the world. But you play it the way you play it, with different people, and your solution is unique. Different things happen to your party than with your buddy's party.
In a way, a really good book series like Salvatore's Drizzt series (especially Streams of Silver and Mithril Hall) is the answer to the hunger pangs for more stories about characters like Legolas and Gimli. Dragonlance is the answer for more stories about hobbits. There's a lot of great stories out there, and while they're not set in Middle Earth, they're more kindred than derivative.
I can imagine a good series of books emerging from Middle Earth, but it's hard not to imagine a bad series of books, too. It's probably for the best that Middle Earth remains more or less frozen in its incomplete and mysterious time capsule. The stories we'll never hear are likely what keeps us coming back to the books and letters and fragments we do have, like archaeologists and antiquarians of a lost land. The mysteries of Middle Earth that Tolkien left us with remain mysteries, and somehow that's supremely appropriate.