I am not very sure to what  kind of fan game  count in this jam. Does the game must obvious what it based on? Should it feature the character or something? What if the game only has similar gameplay / UI inspired by the original game, while everything else are my own original?

You can make an original game inspired by another game/franchise, as long as it's not hard to tell what the game is based on. We are very loose with definition of fangames, but we also don't want people to submit regular indie games that got nothing to do with the whole concept of the jam.