1. Lance
He may still be a Kanto Trainer at the end of the day, but Johto is where he shines the most, because he actually helped you out to fight Team Rocket.
It would be interesting to explore any potential ties that he may have with other notable dragon Trainers like Iris, Drake, Drasna and especially Leon, given that both Leon and Lance use a Charizard that utilises different niches in battle on their teams.
2. Lillie
It would be nice for players to know what she's been up to lately or any progress that she made so far as a Trainer. Considering that Alola is where the idea of Regional Variants were first discovered, she would be a fantastic choice to provide newcomers on what she knows on behalf of Professor Kukui.
3. Alder
Since Iris became Unova's Champion, this would give Alder extra opportunities to travel the world and share his wisdom with others as a way of showcasing an older generation passing the torch to a younger new generation.
It may provide the opportunity to know more about him and his past.
4. Steven Stone
Objectively the most iconic Trainer of the Hoenn region. If any region got hit by the radiation of the Ultimate Weapon's launch or the meteorite that struck Hoenn over 3,000 years ago, he would be the Trainer to explore how it affected the formation of stones and fossils in those regions.