
I think it would have been much better if an adult Ash had become Liko's mentor. The transition from one protagonist to another would have felt far more natural and emotional.
Imagine the story taking place many years in the future. Ash is now an adult, a legendary Pokémon Trainer, and an established Champion. Since the ninth generation is set in a school environment, the Academy could invite him to give an inspirational speech to the students.
During his visit, Ash meets Liko and immediately recognizes her incredible potential. Instead of making only a brief cameo, he decides to mentor her and help her grow as a Pokémon Trainer. Throughout the series, he would serve as a true mentor, sharing his experience, encouraging Liko to find her own path, and showing that she doesn't need to become another Ash in order to become a great trainer.
In this version, she could be a Trainer with incredible potential and talent. Ash could recognize his younger self in her and decide to train her, drawing on his two decades of experience. Maybe she would be the one to ask. Ash would test her, and after seeing her talent and determination, he would decide to accept her request.
We could have flashbacks during the moments when he gives Liko advice. Not just fanservice, but scenes that actually serve the story, recalling his battle against Tobias, the Kalos League final, and the Battle Frontier.
At the end of the story, Ash realizes that his student is finally ready to continue her journey on her own. He says a heartfelt goodbye to both Liko and the audience, officially passing his legacy on to the next generation before setting off on a new adventure.
In my opinion, this would have been a much more organic, respectful, and satisfying passing of the torch. It also would have been amazing to finally see Ash as an accomplished adult who achieved the dream he spent so many years chasing