We need to really stop talking about how its "common sense" or "unlikely" or come up with fake rules to dishearten speculation. The more we come to accept that anything is possible, the better. Because do we have any idea where this anime is going to go other than the few details we've been given? No. Also do you really expect them to not spend a decent amount of time in the new region. We gotta learn about all those new critters somehow. Also depending on how the missions go, Ash can easily spend time in the 8 towns where the gyms are in Galar. There's no way the Galar region isn't gonna get a lot of focus considering its the new one. Plus do we really think they're gonna pass up the opportunity to have Ash use Dynamaxing and Gigantamaxing? I could see Ash still doing gyms.
Well the number 8 does mean "all possibilities" and we're doing all the regions. Also if that theory about them having a teleportation machine is right because I've seen enough sci-fi to say this honestly looks like one.
Getting around will be a lot quicker then before. He can just port to each gym's location. Or areas like mountains or forests to catch Pokemon.
Regardless I believe Ash will still do the Galar League in some form. He's the Battler to Gou's Catcher after all.
And while in the other regions Ash rotates in his pokemon of those regions giving them some time in the limelight again. Maybe he can do the Gen IV Battle Frontier during the "3 months till the Champion Cup begins" after he gets all eight badges. Maybe even redo the World Tournament or even the Indigo League? Anything is possible now right?
There is just the question of if they do have a teleportation device are we just going to be jumping all over the place? Or are we getting something like where each region gets its own story arc one in-between visits to Galar?
With Kanto being the hub location this would let them bring in Team Rocket as the main villain team again because Team Yell are either just pawns/patsies for someone or just overzealous fans instead of true villains. The show needs proper villains to truly challenge the heroes. Also villains who can be in every region. Team Yell can't really do that.
And Team Rocket could perhaps be used as foils or shadow archetypes to Go and his goal of catching them all and especially Mew. Team Rocket after all did make a super clone of Mew. They also want to catch all the rare and powerful pokemon (even if its just to exploit them for profit or world domination). They want to "Catch'em All" for all the wrong reasons.