Personally, I would try and make them as close to the episodes as possible.
I've only written one fanfiction, about my OC (in my icon ^0^) and I actually didn't introduce him until chapter 3.
I changed his design a little, too. I tried to make him brighter and more "pony-friendly" since before-hand he was black-maned and grey-furred Dx
I really didn't want him to look like a Mary-Sue :<
Anyway, but I didn't make him (yep, he's a he despite the muzzle- working on that xD) the main character, but rather a supporting character.
Only problem was the moral- I've been trying to re-watch some episodes to ensure I didn't copy them, but that left me with very few ideas! D:
In the end I "came up" with something very similar to Zecora's (nooooo!!) but I tried to change it a little, to make it seem like I was referring to working with people who may seem scary or who you may not like to get the job done is normally a good way to go ^0^
But, enough about that, how do you guys write your fanfics? x33