Okay if I’ve missed the existence of licensed RPGs for these, I’ll be happy to be proven wrong.
Abhorsen Series, by Garth Nix. I’ve tinkered with making an Abhorsen type character for 5e, but I think a fitd game would kick ass.
The Dark Tower series, Stephen King. With supplements for playing in the odd lands from The Talisman, Eyes of The Dragon, and any others that feel slightly off from Mid-World.
A Court of Thorns and Roses. I haven’t read it, but my wife has and she likes to recap the book she’s reading to me before bed, so I know enough to hate Tamlin.
Different kinds of Fey, different courts with different kinds of magic and internal factions, not to mention the humans, and whatever else.
The Locked Tomb by Tamsyn Muir. No, Thirsty Sword Lesbians does not count. I want lesbian necromancers in space, thank you. But seriously rules for playing a necromancer and they’re cavalier, or a member of the cohort, choosing your house and your place within it. Ugh. Yes pls. I’d be torn between the Fourth, Fifth, and Sixth houses. Or Ninth, just to be maximally goth and sad.
The Galaxy Rangers, from tv. Favorite cartoon as a kid, and a rad setting full of innumerable species of weirdo aliens, with cybernetic enhancements, psionics, and talking robot horses. Space western at its finest.
The combined universe of She-Ra, He-man, Thundercats, and whatever other old space fantasy cartoons were happeneing around that same time. I think there is an unofficial She-Ra game, but not a licensed one, IIRC.
Do a chapter or even a small book on each one, with a core rules book that they all run off of, and that presents The Known Universe connecting them all.
The Borrowers, Thumbelina, Ferngully, the one where they ride on hummingbirds and Mother Nature is Beyoncé? Ithink there are some similar concepts, some where you play mice rather than tiny people, but like…lemme play a tiny person.
Dragonriders of Pern - definitely a pbta game of some kind, as a trad RPG would probably struggle to feel right without being boring. Just such a gorgeous and compelling world.
Anything by Mercedes Lackey, tbh. I really need to revisit her work at some point.
Neil Gaiman’s Neverwhere, Stardust, and even The Sandman since I hardly think generic DC RPGs count. Each presents different opportunities. Mirrormask!
Just any of his weird worlds.
Narnia. Lemme play a mouse swashbuckler pls and thanks.
Prydain. Glorious! It’s just such a wonderful and dangerous world with so much to offer.