I think it should be Kanto again. BUT.

Thanks to the rifts that opened up when the UB´s attacked Alola, the reality/World collapsed.
the Multiverse merged into ONE gigantic world.
Kanto, jotho, unova, alola and so on are merged into one, gigantic world. while the UB´s roam freely through the world, the international police trys to get things in order.
That where you (The SuMo protag) and your friends appear. Since ou were a rifter, the the wormhole energy is onto you, the UB´s aredrawn to you. You need to protect citizens and create something like a task force to stop them.
By "recruiting" a team of protags/antags (Red/Blue/Silver/Chicken/Serena) and important NPC´s (Cynthia/Wally/Looker) you can gain controol of the situation and calm things.

Also there will be a return of team rocket, who finally make their comback as some kind of evil team. though not evil. they try to destroy the ultrabeasts, while you try to save them by capturing them.

at the end it´l be that the king of the UB´s, a humongous monster, will apear. after, y´know, batting against arceus and winning, you and your tagteam have to face him.

there´ll be 198 other pokemon, with 1000 being the big bad and 1001 being the "world" or something. like 40-50 ultrabeasts in total, and mons from other multiverses.

So, thats what i thoguth about at work today. what do you think?

Many new mons, new UB´s, no "Genwunner" since its all merged, and lots of postgame. and so on.
Of course it´ll be for the switch with good grapics and so on

(I know its GF we´re talking about, but hey, let a man dream)