Adventures of the Galaxy Rangers
I'm not ordinarily a fan of the whole "wild west meets space" thing, but damned if this show didn't do it right. The mechanical horses were a bit annoying, but they built an enormous back story that would make a terrific setting for an RPG. The Super Troopers, the Queen of the Crown (and the Crown Agents), the myriad of aliens... it is a gold mine for RPG goodness.
Oh. My. Gods. How did Disney never turn this into an RPG? Magical creatures, mutants, robots, and cyborgs, and Shakespeare (MacBeth-- yes, THAT MacBeth--, Oberon, and Puck are recurring characters, for crying out loud!). There are some epic themes that resonate through this series, and the notion of finding new and unique variations on the "European gargoyles" trope could keep such a game (and game supplements) going for quite some time.
The movies were good, but the TV show was better. They vastly expanded the rogues' gallery with really interesting villains (Abis Mal, Mozenrath, and Mechanicles "the greatest of the Great Greek Philosophers"), and gave the whole setting a lot of heft. You could play an entire campaign in Agrabah and never encounter Jasmine or Aladdin (although you might well encounter Sadira). The quantity of other characters makes it a worthy candidate.
You could make a lot out of this show; different types of cats having different abilities, and there are an infinite number of potential "hideous mutant" types, not to mention the Lunateks. And Mumm-ra is the ultimate recurring villain; defeatable in the short term, but never defeatable in the long term. Add in the Amazons, the Berbils, Wollows, and Bolkins, and there's a decent lineup of NPCs.
Pirates of Dark Water
Ignore the whole "lost 13 treasures" thing; this cartoon had possibly the richest background of any 2-season cartoon ever made. And if you're looking for a template for a maritime-based game, look no further. Noy Jitat! What a game this would make.
That's my five. What show would you like to see turned into an RPG?
Happy 50th, Star Trek - I was just a bit too young to remember watching Trek when it was in first-run on television (but I was alive then, and it's certainly possible that I was ...