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So some of this people already mentioned and some of it might be a stretch, but in the quick flash of Goh's Pokemon, they went by Gen order, in which we also saw:
>Gen 2: Pokemon crawling all along trees similar to OK! (pic related). Suicune also appears in the background (It's Goh's of course, but worth also remembering how prominent it was in the Johto OPs)
>Gen 3: Shot looking at a city at dusk similar to a shot from Advance Adventure
>Gen 4: Dawn shows up
>Gen 7: Ash's Alola Team outside the Kukui home base
I feel like this is going to be the part of the OP that we're going to get a lot of small edits to over time.