> So it's not going to tell the main story itself, which is probably wise given the film adaptations aren't even that old.
As far as my understanding of the very complex and sad story of LotR rights goes, they can't/. In a nutshell, JRRT has sold all non-printing rights for LotrR and Hobbit a long time ago, and they are currently held by the entity (currently) called Middle-earth Enterprises [1], a division of The Saul Zaentz Company. This basically boils down to them having the final world on any derivative work that includes the characters, locations and some other elements from the novels.
The rest of the rights belong to the Tolkien Estate [2], which is a legal entity controlled primarily by Tolkien's son Christopher. The Estate have for long been opposed to any derivative works, but they have apparently changed their stance, considering they are the ones Amazon made the deal with.
So I expect we won't see any of the Fellowship, nor places like Rivendell, Moria or Minas-Tirith; but there are plenty of areas in Middle-earth with enough stories to explore -- early days of Arnor and Gondor, Dol Amroth etc.
[1] https://en.wikipedia.org/wiki/Middle-earth_Enterprises
[2] https://en.wikipedia.org/wiki/Tolkien_Estate