It appears as though Pokemon Ultra Sun and Moon will be the last of the Pokemon RPG entries to come be released on the Nintendo 3DS before the franchise makes it's way to the Nintendo Switch.
When speaking to IGN, Game Freak director Shigeru Ohmori stated that with Pokemon Sun and Moon, the company believes they have pushed the handheld as far as they feel it can go:
"When we were making Pokemon X and Y, we really were trying to push the 3DS system to its absolute limits - which is what we thought we’d done. But when Sun and Moon came around, we completely redesigned the system, and actually ended up pushing the 3DS even further to what we thought was the most we could draw out of it."
Mr. Ohmori continues, stating that with the Ultra versions of Sun and Moon, Game Freak have done their best to squeeze out absolutely everything they can from the device, meaning that after these releases in November, the company will be able to move on:
"With Ultra Sun and Ultra Moon, we’ve tried to eke that out more and really, really push the system to its absolute limits, and we’re now feeling that perhaps this is the maximum of what we can get out. So we’re really treating Ultra Sun and Ultra Moon as the culmination of our work with the 3DS system."
At this time nothing is know about what kind of Pokemon game will be coming to the Nintendo Switch, nor how many of their staff are working on it, however, Kazumasa Iwao, the Game Freak director of the latest 3DS Pokemon games, explained that the "comparatively young team" working on Ultra Sun and Moon is at 80 people, approximately half the size of the Sun and Moon team.
What we do know is that the Untitled Switch game's development is taking into account "better visuals, multiplayer capacity and extra accessories."
Pokemon Ultra Sun and Moon release to the Nintendo 3DS on November 17, 2017.