Hmmm...
Avatar: The Last Airbender/Legend of Kora: Fantastic, rich setting with martial arts and elemental magic. I have no idea why this hasn't been done more as a game
Wild Cards: A different take on superheroes; maybe it will get more love if GRRM actually does a TV series
The Man In The High Castle: Alternate history, espionage, and resistance
Tekumel: It's a good book series and one of the original RPG settings. Rich in history and detail, clan politics; a great non-Western fantasy series. I think that it's the level of history and societal detail in the books that prevents it from being used as a game, sadly.
RPG's
Top Secret SI: There should be more straight up espionage games, and Top Secret SI (the second edition) had a fantastic set up with clearly good guys (The Orion Initiative) and bad guys (WEB), instead of murky realpolitik. There's a new edition of the game coming out soon by the original creator, Merle Rasmussen.
Eclipse Phase: One of the best sci-fi settings, game or otherwise, in years. Doing this game could combine various player's cravings for an 'Expanse' game and a 'Cyberpunk' game in one.
Blue Rose: A nifty fantasy RPG from Green Ronin; the first edition used their True 20 system, the new second edition uses their AGE system. A romantic fantasy game where the emphasis is on relationships and working as part of a group to protect society, this is pretty much the 'Chronicles of Valdamar' game that fantasy readers have been asking for, but Mercedes Lackey has been quashing. Plus, you can play an intelligent, psychic animal. The setting might be a little too accepting of gay and trans characters for the tastes of some hardcore gamers, though (the main kingdom has full equality and acceptance of gay/lesbian persons, and second edition adds ideas like using magic to shape new bodies for transsexuals to complete their transformations. Also, Intolerant Monotheistic Theocratic Frenemy Country on the border that's only not a full enemy because of Evil Necromantic Nation of Evilness to the North). Still, it's a good break from the traditional 'murder hobo' adventuring of the majority of fantasy games, and the emphasis on social relationships fits in nicely with what the majority of MU*s are or turn into, IMO.
Scion: Wish this game would have gotten more love from the MU* community. I played Monsters and Moshpits while it was around, and Scion remains one of my all-time favorite games. Hoping 2nd Edition turns out well, but feeling nervous after Onyx Path pushed out Exalted 3rd...
7th Sea: Again, another fantastic RPG that never seemed to get love from the MU community. Swashbuckling, magic, deep conspiracies...this game was a blast. Sadly, I'm not too thrilled with 2nd edition, but the setting is still great.
Renegade Legion: Outshined by it's big brothers at FASA, Shadowrun and Battletech, Renegade Legion had a great premise: high space opera focusing on a group of resistance fighters against an overwhelming and evil galactic government. If it sounds familiar, it's because Renegade Legion grew out of the game design that FASA pitched for a Star Wars RPG license that ultimately went to West End Games. However, changes were made, like the Evil Galactic Empire rising from the ashes of Earth and being modeled on a fascist interpretation of the Roman Empire. It was fun, had lots of emphasis on epic combat, and had a great deal of setting depth and lore.
TORG: Speaking of West End...Torg was just an awesome expression of 10 gallons of weirdness in a 5 gallon hat. America converted into Dinosaur Land. The British Isles converted to a fairy tale kingdom with an epic story of Light versus Dark. The South Asian island chains turned into horror-scapes with Victorian Colonial 'White Man's Burden'. A French papacy mixed with cyberpunk where people experience God and Heaven by jacking in...The setting was amazing and the system was enjoyable. The card mechanics shouldn't be too hard to duplicate. If you ever wanted your Japanese Techno-future corporate ninja to have adventures alongside a fantasy paladin, a werewolf, and The Shadow, this is the game for you.