Let's see, I think my first tip about fanfic writing overall (I'm not saying I've always lived by this idea myself BTW, you should see some of my older stories) is:
When thinking outside the box about how a character will react to something new that was never covered in the series/show itself, be careful to keep them in character. Even when someone grows, unless something very scarring happens to change their outlook, they are basically the same person/animal/thing/whatever.
True emotion in a fandom story happens IMHO, when the fans that read it can see whatever you are writing actually being a real reaction of that character, even if it is a crazy/dramatic place you put them in
But seriously, if I see one more different world fanfic with
author made characters who only have
canon character names set in
high school I will scream!