There could be a chance that later on, we could be seeing something like California or Okinawa-like addition. Maybe the Plus/Minus rumors were right on the fact that other parts of the game are based in Southeast Asia, a location controlled by France via Vietnam and Cambodia. Even if I'm wrong on that end, any connection to Kalos doesn't have to be necessarily on the mainland. We know so little about Kalos that they could be a colonial power, where it can still be another part of Kalos we haven't explored. If we're really relying on airplanes for travel they could go to any of the other parts of Kalos' control without issue. We could even get Morocco, Algeria, Egypt or even Syria and Lebanon. It depends on the time period they are aiming for. I doubt a French occupied Mexico would work.
Maybe the reason why Alola is so small is because the rest is more vast. If they are even going to Kalos once more , they'd have to redo all of it in the newer style anyways. But Gamefreak has been bucking traditions since Black and White so anything can be possible when it comes to these games.