Pokémon Ranger - if implemented well with the Joy-Cons - would be interesting. Especially if they can do open-world properly.
Another Legends - although I haven't completed the first one - would also go down well. I'd love one for Hoenn - perhaps with Arceus again pulling the strings, as like an overarching thing for the series, but Rayquaza would be the final boss. They could truly embrace Hoenn's tropical environment in full 3D, which would be awesome. Have Tropius as the flying ride Pokémon, Sharpedo for surf, Sableye (that you don't ride) for the item-finding, Vigoroth? for scaling walls, and Manectric for running around. Or use the starters - Sceptile can climb walls and trees!, Blaziken could charge around with you on its back, and Swampert could be used in Water.
Or, given my fear that they'll give it the BDSP treatment and keep the sprites etc. I could just greenlight a fully 3D remake of Black and White, with B2W2 areas added in post-game (perhaps as an expansion of that Looker quest, Ghetsis' revival being stopped whilst the ship is still under construction!).