Good SF is original and imaginative. SF can include romance and adventure, but what separates it from these is the way in which the world either changes or has already changed.
So, in Greg Bear's Forge of God, the entire world is destroyed. SF helps us to encompass ideas that are otherwise too great for the human mind.
Some authors love writing fan-fic, and a tried and tested formula is a good place to start writing. However, in order to be published, you need to move beyond existing characters and develop your own universe.