This thoery isn't a complete impossibility. After all, Togepi and Togetic were introduced with Misty. Should Togekiss make an anime appearance, it would make sense that Misty would return again.
While Misty coming back for something being involved with her Togetic would be better than just waiting for Ash in Pallet to say hi,at the same time as far as i know the only portal which leads to Mirage Kingdom currently known is in Hoenn,so this isnt very likely though i wont say its impossible.
As for Misty being adamant gym leader as its common knowleadge her goal is to become best with water types(water master) so her leaving the gym for whether its shorter or longer period of time makes sense to catch either new water types or to compete in some water competitions.Nevertheless as it has been showed/implied the main reason for her not leaving the gym are her incompetent sisters.
With that being said when talking about Mistys appearance(not counting return which unfortunately isnt very promising),it is going to be something along these lines:
-Being located again in Pallet(whih unfortunately has the biggest chance of happening)
-Coming back to participate in some water tournament like Whirl Cup or something(chance exists especially if next Battle Frontier(assuming there is any) is being placed in Johto.
Return to participate in contest(this isnt very likely because contests arent her thing,and if she would ever entered one it would be mostly just to see how its like.)
-To cheer for Ash during Sinnoh league,while its possible at the same time from what i noticed this wouldnt satisfy people too much.
-Something being involved with her Togetic and evolution in Togekiss(many people say its very unlikely,but this option cant be entirely written off).
-Group visiting her in Cerulean(chance exists)
-To showcase some new water type,whether legendary or common one(maybe).
-Showing up and being involved with something with Team Galactic(some fans speculated about this but this isnt very likely).
-Something totally unexpected not being connected with any of above mentioned scenarios(not sure what).