Kalos Queen is just a title right? She wants to be a great pokemon performer which she can do in Alola as well.
Also, if the writers leave Serena in kalos, we would not see her dream progress.So, literally, there is no use of leaving her in Kalos unless they want to dump the Character.
But, i don't see her coming to Alola.
Reasons She should come to alola:
1) She has only 3 pokemon
2)Poppolio might be perfect for serena
3) In an episode Serena asks ash whether they will be together forever and Ash says yes.
4) Alola looks like a fun place
5) They could easily give serena a new costume of the female Protagonist in SM games.
6) Fan favorite
7) Cares for ash.
Kalos queen is not just a name-only title , It present the queen of Kalos in Showcase.
Serena is asked to be an apprentice of Palermo who happen to be former Kalos queen and mentor of current Kalos queen .
So , We can see Serena most definitely becoming Kalos queen in future.
Serena's position is far better then Misty , May or Iris ! We don't know if any one of them is near to fulfilling their goal.
In fact , We don't even know which region May gone after Jotho or If she ever become Top-coordinator !
1) Tracey had only 3 pokemon but he didn't joined for Jotho ! Plus 2 pokemon is enough for a Pokemon performer but Serena got 3 pokemon.
2) Popplio is even more perfect for Pokemon Contest.
3)That doesn't meant anything ! Ash promised that he will come back Pidgeot but he never did.
4) Alola is much better place for Misty since its a tropical region .
5) But Serena won't look that good!
6) Serena is not as Fan favourite as Misty or May ! She is just popular with Amourshipper.
7) Every Female Companion cared for Ash! Serena is not special just because she hold one-side affection.
8) Pokemon Showcase completely Boring! It less of a competition and more of a Fan-service of Guys ! I rather prefer the Anime bring back Contest since it has everything Showcase has plus Battling. In fact , All Member of this form voted Contest to be better then Showcase.