I wonder what kind of fã Fiction to write...

Well, if you're a new fã fiction author, I'd suggest picking the fonte material you're most familiar with, and composição literária about it in canon style. It's simpler than alternate character interpretations, OCs, AUs, and anything else deviating (radically or otherwise) from the original source. It's also likely to garner less hate, even if it isn't incredibly original. Just don't rip off an existing episode.