Nintendo has been dealing very well with Open World, it seems. Despite it being cancerous this generation, as FPS was for the last one, they are greatly proving that it is just a matter of effort. Despite that, I feel like Pokémon is the RPG that needs it the most. The multiplayer is great because you can be unique. Your team will always be different, there are way too many combinations for one person to be similar to another. Add on that little touches as avatar customization, PokéBall seals, poses and the like, and you can always make your own identity in Pokémon. So the games were never just a single player experience, even if they can be. But the single player experience is as fun as playing Animal Crossing, for example. Grooming, growing, finding, doing special time-related events, seeing the overall world. IMO. I never saw Pokémon as an RPG. A RPG experience where you go for the plot and overcome challenges.

Neither do I see the story as something that has one start, middle and end. The fun from Pokémon's "plot" comes from meeting unique characters and events that add up into a bigger sum. So I think that having a very well fleshed out world to explore and finding Pokémon in new ways would be ideal. Being able to climb, crouch and enter in tight places, swim, shake things, seek hidden pokés is what I expect for a long time, as well as no random encounters, the trainer having its own level, gaining experience by capturing a milestone of pokés, beating gyms and other specific activities, and unlocking abilities as increased levels of experience won for your pokés, training them faster, increasing chances of capture, increasing friendship rates, learning field skills (being able to reach higher places, being able to shake bushes, trees, etc), increased EV training efficiency, being able to ask for rematches, increased storage numbers, increased shiny rate, etc. are all features that would help the series leave its mark in the genre.