Dixie as an alternate custome is like making wario a mario clone, is just not right.
If gender swaps are implemented for all the characters we will get a new diddy girl form, not a dixie costume.
The most plausible end outside a character appareance is an AT, a background element or a palette swap for diddy, but never a gender swapp.
I think it had nothing to do with the avatar things btw.
The fit trainers arent avatars, they are "trainers", your instructors, you use the miis in the game.
Male trainer was supposed to be a different character, but because of the abbilities of both trainers is the same he is an AC, something really odd in my opinion but i guess it works.
So, equal characters with different gender could be considered as an AC, like lucina i suppose, but again, dixie dont enter in that definition.