In a Pokemon Let's Go Pikachu and Eeve interview:
>hat would you say to those players who are crying out for a bit more of a challenge, maybe adult players who've been playing for these 20 years or who got into it a bit later? Should they wait for next year's RPG instead?>Kensaku Nabana: And also the postgame content as well, in particular the master trainiers - so these are trainers who are kind of the ultimate trainers of a specific Pokémon, so you will challenge them. So you will challenge them, and then get their title for that Pokémon, so you become the master of that Pokémon if you manage to beat them in battle.HOly fucking SHIT! We have Pokemon masters now! Is this what Ash has been working towards!?
Interview
https://www.eurogamer.net/articles/2018-10-12-junichi-masuda-on-pokemon-lets-gos-difficulty-meltans-reveal-and-the-futureNintendo Soup reveals master trainers
https://nintendosoup.com/masuda-reveals-master-trainers-for-pokemon-lets-go-pikachu-eevee/ 