I'm gonna say ThunderCats, because the rights for it are basically up in the air. It should have gotten a film adaptation or a franchise like Masters of the Universe or Transformers, but nobody can figure out who owns it, so they can't make a movie without some rando suing them... so they don't.

The OG came out in 1985 and ran for something like 150 episodes, later divided into four seasons. But back then it just came on TV, it wasn't numbered or divided in any way (aside from multi-part episodes). They did a remake in 2011 on Cartoon Network that was pretty cool, but they couldn't get the legal stuff in order to get a second season — it was absolutely popular enough.

15 years later, I don't think anything's been done with it outside of, maybe, fan fiction and fan art.

Going back even older... Mysterious Cities of Gold. That's never been remade or adapted... however... while Tomb Raider was basically "we want to make a video game of Indiana Jones but we can't get the rights, so let's make it a girl with big tits," for some reason, they wanted to swing it back the other way and say "let's make Tomb Raider but have it be a hot guy instead and keep it exclusive to the PlayStation because at this point the concept is just so watered down, we'd lose money if we ported it to PC/Xbox" and called it Uncharted. Then they said fuck it and made an actual Indiana Jones game. Le sigh. But anyway, I wanna say one of the Uncharted games leaned into Mysterious Cities of Gold territory... but I'm not really sure. Anyway, it was about three Mexican kids (I think, as in born & raised in Mexico) who seek out the eponymous golden cities. IIRC they do find the ruins of the city they were looking for, but they expected the cities to be actually made of gold. And maybe they found some gold but not like they thought. But they had a plane and I think they had some cool adventures, so they broke even I suppose?