This is how the legacy of Ash Ketchum should carry on:
Ash and friends should be aged up to young adults who have achieved their dreams.
Liko and Roy are big fans of Ash and hope to meet him someday.
On their journey, they meet big names in the Pokemon world (Ash's friends) who tell them all about their journeys with the legendary champion:
Misty is pissed that her (boy)friend hasn't come by or called for a long time.
Brock helps heal their Pokemon and is recognized as Ash's best friend.
Tracey shows them sketches of Ash and his Pokemon throughout the years.
Gary sneers that Ashy boy is just another trainer, but obviously doesn't believe it.
Professor Oak says they remind him of someone he knows.
May, Dawn, and Serena tell them how much Ash taught them about friendship and battling, and how he taught then to chase their dreams.
Iris is mad because she lost a battle with Ash who used an ice type against her.
Cilan tells them how perfectly matched Ash and Pikachu have always been.
Bonnie and Max, who now travel together as teens, talk about how heroic Ash has always been.
The Alola gang talks about how they went to school together and how he won the first Alolan Championship.
Professor Kukui reflects on how Ash became like a son to him.
The stories make them want to meet him even more!
However, he is very elusive, and almost no one knows where he is at any given time.
Eventually, they do bump into him briefly, but he is busy saving the world as usual.
They don't get to talk to him much, but he inspires them to chase their dreams even more.
They continue their journey, hoping to meet him again someday.
I know they don't want to age him up, but think about how well Legend of Korra went over! They even killed main characters, and the world of Avatar carried on.
Pokemon wouldn't have to kill characters off, but aging them up would be a nod to the older fans and give him ways to fit into the new series in a new way.