Oh you dont have to worry about that mate, i think Iris and Cilan already proved how writers got better since Johto days(and they aren't coordinators).
And writers always dud better job with shorter sagas like Kanto, Orange or Hoenn showed, and after so much time you can bet they would plan out arc through which we could see sequel to Misty story being done.
As for May joining for filler arc , i doubt Rs remakes are going to happen after Unova league considering how Nintendo and Gamefreak only released sequel to BW games, with writers having only this to work with before new games come out. Not to mention since there are no contests in anime anymore, it would be hard to plan out sustainable enough arc which would support May long stay.
Until RS remakes get made, May probably wont return.
But when talking about BW filler arc after Unova league, Misty would be suitable candidate to travel with cast for awhile, not only because of marketing value playing important role as participant of world tournament, but also because by entering such tournaments would help a lot to advance her water master goal and become strongest water trainer in world. Not to mention since pokemon from Unova isn't possible to obtain in other regions, is one more reason to support her journey over there.
p.s. And yes your right, Misty deserves more. Her story has loose ends, she could use more development and even after 11 years people still want her back, being very, very popular.
I think it would be cool if she followed Wallace footsteps one day, going on quest to learn things she cant at gym becoming stronger, and become one day E4 of water types(or her idol Lorelei apprentice). That is if sisters become responsible allowing her to leave freely, being main reason behind stay.