If I am remembering correctly, I think Gamefreak said in a fairly recent interview that they would be open to including more than one region in a game, if they can think of an interesting and exciting way to do it.
Personally, after US/UM I'm not overly keen. These games seem quite cluttered with references to past games - which is great in these games, as they act as a culmination of what has come before them. But with the move to the switch, I am really hoping for a fresh new start. Respecting what has come before, but primarily looking forward. I think adding an old region in would take away from that.
Having said that, if they were to bring back past regions, a way they could do it without making the games too long or bloated, could be to just bring back familiar towns or areas from previous regions that you need to travel to to do something. That way you would technically travel to the region, you just wouldn't have the entire region open to you.
One plot idea that I have always liked is having the whole in-game world threatened by some all powerful force (e.g. an ancient evil pokemon, or an indomitable enemy team) and requiring the hero-character to either build a team of other strong trainers (which would require them to travel to previous regions, battle familiar/popular gym leaders, and then recruit them) or track down a mighty legendary pokemon to help you. With that second option, clues pointing to the location of the legendary would be scattered across all regions of the world (either fragments of a map, or cryptic clues from people) and you have to revisit familiar locations from past regions to uncover those clues.
