Bring Back National Dex - Here's a possible solution

They were able to "cram" every Pokemon into the code of the previous games in every game before it.The only exceptions being new forms or Pokemon introduced mid gen.
Not only that, they did it on inferior hardware than the Switch. They only had to create all the models once, for XY and have reused those same models since. There are no excuses

And I can live without another stadium game if it's gonna turn out like the abomination that was Battle Revolution

Something tells me that the original 150 will all somehow be "appropriate for the region."