Stardust! Pick you medium, it started as a graphic novel, became a book and then became a movie. Fond of its earlier incarnations though I am, its the film version that I'd like to talk about.
Stardust is the story of Tristan Thorne, a shop boy from the village of Wall - a sleepy little English village which guards the gap in a wall which divides the realms of England and Stormhold. Tristan sets off to bring back a fallen star (in order to win the heart of Victoria) only to discover that, because Stormhold is a magical kingdom, the fallen star is not a lump of celestial rock but rather an injured young woman who isn't going to take being kidnapped lying down... However, Tristan is the least of her problems with a trio of witch queens after her heart, some blood thirsty princes after her necklace and some fearsome sky pirates along for the ride the race is on.
The film is a much lighter creature than the book version, clearly aimed at a much younger audience having more humour and swashbuckling, along with a more hopeful ending. The fandom is pretty tiny, it was bigger around the time that the film came out but as that was the best part of three years ago it has noticeably trailed off. There's plenty of humour and high adventure which makes it a great fandom for gen fic as well as romances of pretty much every variety. Apart from the central duo of Tristan and Yvaine who have an excellent line in sarcasm and banter, there are a plethora of supporting characters to write about and explore.
Tristan's Parents: First off we have Dunstan (a young scientifically minded man from Wall) and Una (Princess of Stormhold and unwilling captive of Ditchwater Sal) who haven't seen each other since Tristan was conceived but may get a second chance now he's grown up.
The Lamia: Witches seeking eternal youth, they're after the fallen star and will stop at nothing to get what they need from her.
Skypirates: Fearsome fighters of the air, collecting and selling lightning on the black market, led by the fearsome Captain Shakespeare (a man with a blood thirsty reputation and a fondness for classical music and cross-dressing).
Ghostly Princes: In order to ascend the throne of Stormhold the heir in question must kill off all their competitors, not easy when there are seven brothers all as bloodthirsty as each other. Unfortunately for the dead siblings they have to stick around as a ghostly greek chorus until the new King is established.