A Fighting-type Gym Leader
It’s well-established that among fighting-type enthusiast trainers, sparring with one’s Pokemon is a very common event. So, it wouldn’t be completely illogical to include one trainer as a playable character. If a trainer were to be included, it would most likely be one of the fighting-type Gym Leaders from the series, as they would be far more recognizable than a generic Black Belt type trainer. Considering also the wide variety of Gym Leaders there are on display throughout the series, it would be easy to take any number of the Gym Leaders and create a move-set for them based on what type of fighting they do; Korrina from X&Y could have a move-set based around roller derby, for example, much like her gym was in the games.
Odds of Inclusion: Middling To Low