If Misty returns she could come for some tournament(similar to Whirl Cup) to advance her skills as water trainer(her goal is to becme water pokemon master so this makes sense).Also Considering how Unova is described as very isolated region being far from others means its hard to get pokemon from there in other regions so coming to catch some new hard to get water type is also posibillity.
Or she could come to help contribute to someones story like helping Iris about possible gym issues having things in common with her since she is gym leader too(similar how May helped Dawn to get over selfesteem problem).
Speaking of gym,i dont think thats really a problem considering how it didnt stoped her from coming in Hoenn.Problem of not having anyone to leave it in their hsands could be easily solved by making one of sisters more responsible(Daisy is best bet)or introduce someone else(like unown relative)to take care of it.
If Fantina,Jasmine,Wallace etc could all leave gyms to improve as trainers or for some other reasons there is no reason why Misty couldnt aswell.
Sorry if this was off topic or anything,just wanted to share some of my ideas or suggestions with others in here.