i2i
Big Bad Wolf
This is my own personal theory based on the fact we haven't got any news on the next series yet but think there planning a sort of Hisui arc.If the time machine becomes a heavy element in the plot, I'd like to see the hisuian pokemon join the fridge too, but, let's see.
Only way I see Ash with past gen starters is either the next series is still doing the world tour like JN or they receive new evolutions or regional forms.Of course there is the possibility that the outcome could be a mix and match of some of these like for instance he gets both a past gen starter and a gen 9 starter and even brings one back.
Or he gets 2 or 3 past gen starters by either catching them and/or bringing them back.