The allure of Misty returning is something fans have been discussing for at least a decade now. Of all the former Pokegirls, and considering SM's setting, it would make sense to have a Water Gym Leader pursuing her goal of finding, catching and mastering Water type Pokemon. I mean in all the seasons, Misty was at home in the Orange islands because she was surrounded by Water. Anyone see the parallels with SM? There just happens to be one major problems with Misty returning.
That problem would be how Misty breaks free from the prison that is the Cerulean Gym? Do her sisters finally take on the responsibility so that their younger sister can pursue her dream? Given that it's been roughly 15 years since we last saw Misty on the anime, it's hard to argue that her sisters have changed to the point where she can even make a cameo appearance(much less join the show for a considerable amount of time) since her sisters are completely incapable of running things(hence she always has to go back).
Granted, I'd love to see her again, even if she's there for a vacation(which is how Ash got there in the first place). Considering the region is a hot spot for Water Pokemon and is a vacation paradise, it'd be an ideal vacation spot for Misty. Tack on a potential rivalry with Lana or a friendship forming into a Master/Apprentice relationship, the potential for Misty returning is indeed there. Heck there is potential animosity with Lana if she sees Lana scolding her younger sisters(Might hit a raw nerve with Misty).