I think I'm pretty disappointed that Delia didn't stay in Alola. For one thing, the winning a free trip to Alola part felt kind of pointless considering that she could just visit whenever. Also if the series could have focus more on individual goals than SOL, then we could've seen what I would consider one of the more unique rivalries throughout the course of the anime, the one between parent and child. It's been stated before that Delia has the potential to be a great trainer and that her personality closely mirrors Ash, and I would've loved to see her travel across the region and develop a team and actually become a rival.
I know that lot's of people love Kukui, but I felt like the show only tapped into that father/son bond whenever it was convenient, and that it conveyed the message that Ash was a child being raised under the guidance of an adult. I would've preferred seeing the parallels between Ash and his mother throughout the series as trainers and watch them learn from one another, maybe starting off mostly with Ash guiding his mother, but then as the series progressed we could see Ash develop a newfound respect for her.