Take a Look, We're in a Book
Sort of like a Pagemaster/Gumby/Wishbone(?) deal. The PCs are able to enter and adventure within the realms of books. Fanfiction: The Game. The players might step into the role of someone in the story, or just be ordinary folk with this singular extraordinary ability.
Permutation 1: What's the deal?
A. It's Real. The players magically enter the world of the books. The dangers they face are real, and physical. Maybe they can bring things into (and out of) the worlds of fiction they enter.
B. It's mental. The players are sharing a hallucination of sorts, or a psychic gestalt. There's less physical danger, but just as much mental stress, and nothing can go in or out (obviously).
C. It's virtual. Technological. Virtual reality, or some kind of holodeck-ish augmented reality game.
Permutation 2: Who do you want to be today?
A. Step into a role. Players become one of the characters in the story. Maybe the protagonists, maybe secondary characters, maybe just background figures who "fit" but with the freedom to act.
B. Just who you are. You're still you. Through your experiences in the books, you gain experience pretty quickly. You learn things you can apply to the next book you enter.
C. A whole new you You're you translated for the genre of the story. Stronger, faster, maybe you have magic, powers, or can fight. This makes it harder to learn new things, though, because of these crutches.
Permutation 3: What media
I'll poll the chosen players regarding their favorite media, and draw from those lists. The fun part here is player familiarity with the intellectual properties. On a larger scale it might be books, comics, movies, television series, video games.
Permutation 4: What system
Something that supports a variety of genre and a spectrum of power levels from the ordinary and mundane to the superpowerful. Ideally something that allows for character growth so the PCs can improve from their adventures. Off the top of my head: Fate or the World of Darkness might work, but feel free to suggest other possibilities.
If you're interested, weigh in on which permutations seem to hold the most potential here.