They could obviously just start making up a whole bunch of stories, much as Shadow of Mordor/Shadow of War did. Btw I will die on the hill of those being actually not-terrible in terms of Tolkienian inspiration and more in line with the morality of the books than the vast majority of Tolkien tie-ins, including virtually all the videogames.
Indeed that's what I expect them to do. Honestly it could be fine. Or terrible. Depending entirely on what they do. I'd kind of love to see someone fighting a guerrilla war in the Southlands against whatever Sauron-controlled leadership they have there (a plotline a Shadow of War expansion had I think), for example.
Why care about Tolkienologists? I say that as someone at least Tolkienologist-adjacent. There are like, dozens of us, worldwide, and we're not the kind of people who can convince a bunch of people to watch your movie/show, or even convince them not to if it sucks.
Sequel and prequels. Lord of the Rings Expanded Universe. Follow the exploits of King Aragorn in the Fourth Age. Witness the rise of yet another threat to the people of Middle Earth. Thrill as the wastrel grandson of Aragorn is forced to confront his legacy as he is tempted down the path of the Dark Side by Darth Krayt.
I mean, it will almost certainly be garbage. Garbage with very nice production design if they use Jackson's people, but garbage nonetheless.
While I don't want to further amplify the awe that the "big worldbuilding fantasy" authors, and Tolkien in particular, are held in, I think fundamentally once a vast, intricate fantasy world is built around someone's singular vision (which even they struggle to keep straight), to make good fiction in that world you either need that person at least in consultation or else you need to do everything with painstaking care. There are just too many moving pieces with obtuse purposes for some other author or screenwriting to come in and bang out something on a deadline without the worldbuilder's supervision.
I don't think it would be hopeless to make a good, single three hour-ish Middle Earth movie not based on any established Tolkien narrative. But the process has to start with a screenwriter taking their long sweet time. The problem is that, since Lord of the Rings is the famous Trilogy, and the ultra big budget side of Hollywood thinks only in cinematic universes now, the idea of a single movie doesn't even occur to them. I has to be more trilogies. And at that point the scale is too large for anyone to take the necessary time to do it right, and to make actual sense of the world they are writing in. This is the problem with a Middle Earth television series as well. And even if they have an actual Tolkien story at the core their going to stretch it into a trilogy like they did The Hobbit, and fill it with crap.
Once again, I'm not trying to build the mythology of "Tolkien the visionary author" any further than it's already been built. But the slow work of decades for the Oxford professor is not a reasonable thing for a team of screenwriters on a deadline with a "make another trilogy" mandate to try to replicate.
Also much of the appeal of Tolkien's work is in how personal it was to a singular eccentric author (which is part of why that author is himself such a focus of obsession for some people). I mean, I'm not a Tom Bombadil fan, but the fact that the work is full of such things that meant something to the author but not necessarily much to most readers, and serve little obvious narrative function, but perhaps serve some other function, is part of the charm.
I could get behind another The Hobbit trilogy, provided it was lovingly and painstakingly remade backwards, frame by frame and entirely in complementary colors, in order to exactly cancel out the travesty that was the initial trilogy.