There is nothing I hate more then a Fan Fiction that has its timing messed up. I have read multiple Fan Fictions where the author just decides to mix to totally different times and puts them together hoping for the best. Another thing is when people write a Fan Fiction shipping two people, when, because of the timing one of the characters is old enough to be the others grandparent. If the timing isn’t right you loose your readers.(Unless the TARDIS shows up and takes the person to the Future/Past but that is another story for another time). One of the key things when writing a Fan Fiction MAKE SURE THE TIME LINES UP!
Also something else that is the hardest to do is keep your character the same. DON’T change the character. For example I am writing a Doctor Who/Hunger Games Crossover Fan Fic and Katniss,though she is a few years older, has a VERY similar attitude and personality. Same with Peeta. You need to keep the character as ‘in character’ as possible or it will be like writing a new story with characters who happen to have the same names as one from the said book/movie.
So keep your times straight and your characters as ‘In character’ as possible and you should have a great Fan Fic….Oh yeah…watch you grammar and spelling too 😉