I recently said this in another form, so here are my thoughts on the anime moving foward.
Some fans may disagree, but having been shown the most latest interview involving Mr. Yuyuma addressing Ash's goal of being a Pokemon Master, I'm now convinced that the show will soon address this enigma eventually, using the same concept of how Ash inspired a Child Kukui to come up with the idea to create an Alola Leaugue Tournament, based on hearing all about Ash's description of how trainers connect with their pokemon through battle tournaments.
If the writers plan to follow something similar, by making a child like dream a reality within the pokemon universe, especially if the concept of Pokemon Master does not officially exist, I'll assume that Ash will accomplish some type of feat that no trainer or very few had ever done before him which will get him a mass recognition to dub him the title of Pokemon Master.
Heck, I'll even liken the above scenario to how in the story of One Piece, the title of Pirate King was nothing but a silly dream that was made up until Gol D Roger attained the One Piece which was the ultimate treasure no pirate had ever claimed, which officially made the Title of Pirate King real.
With that said, its seems highly possible, based on Mr. Yuyuma calling the title of Pokemon Master "a silly child dream", that Ash came up with the concept of being a Pokemon Master after reading some fictional childhood story or legend he found out about and wanted to make this fiction into a reality.
If I were to guess a scenario of how Ash's goal of being a Pokemon Master would correlate to him going to Gen 9 Paldea region, it may in fact involve Ash being invited as special guest to participate in a special training course at the Paldea training school to showcase his skills to the professors in order to help train students.
Because Ash is now a world renowned trainer, I'm assuming the Paldea professors will want to know more about his adventures, and goals, which Ash will end up introducing the idea of wanting to become a Pokemon Master to the Paldea Professors, whom will eventually get the idea to develop an official school and tournament which will allow pokemon trainers around the world the opportunity to attend in order to become a real Pokemon Master.
Since Ash is now considered the best trainer in the world, it would make the most sense for world renowned professors, such as the ones in Paldea, to take his idea concept of being a Pokemon Master, literally, and want to make this child hood title fantasy a reality. In my opinion, this seems to be the most best case scenario to making Ash's dream a reality.