...possibly. I think such a game would inevitably disappoint and annoy a lot of people though, because it would almost certainly be a live service title, or worse a F2P one in a similar fashion to Pokemon Go. I can't see Game Freak or anyone else making and releasing a game with all regions in it from the get-go - the amount of work that would require would be staggering, and even if the budget could be found it's really not worth it when they can just keep half-arsing the games and raking in the same kind of money - but I CAN see a game in the style of the current anime being released in the future and frequently updated with free and paid content that consists of other regions and Pokemon. Such a title would be amazing if handled properly, but given the greed and laziness that drives Pokemon and such live service titles forward, I can't see it being the kind of dream that people would think of when they think of a Pokemon title that spans every region.
So it's possible, but let's all hope it's not going to happen.