ShinyCharyZard
Too old for your rubbish..
As I've said before, I think every traveling companion Ash has ever had will appear in this series. Next year, the anime is turning 25 years old, so having them all would be a good way to celebrate. So far, we have most of the Alola gang, and now we have Iris. I'm willing to bet Serena is next.
I wouldn't be surprised if Dawn comes next, either instead of Serena or alongside her. She was definitely a more popular character (I prefer May overall, but thats unlikely to happen) and if the rumours are true about a remake, then it would make sense. They could also opt to bring back Misty and Brock before either Dawn OR Serena, since its the 25th anniversary and they are the original money makers for the anime..