Expand on their universe in an interesting yet believable way, but also stay true to the original source. Try and do something a little different than the norm, have fun with the fiction genre. I see too much of people doing a lot of the same stuff in fan fiction when being a little adventurous is much more fun to read in my opinion. I wouldn’t take Tododeku story as an example of being adventurous. I was like 12 yrs old lol, thanks for the support btw <3