So you probably are unsatifated (sp) that Let's go Eevee and Let's go pikarat didn't have generation 2-7 pokemon (minus the alola forms and Meltan) well after looking here https://www.youtube.com/watch?v=lJQZi-0NNHk one fan (?) asked mr. Masuda "Any plans to add pokemon from other generations", mr. Masuda was a pessimist but said that if the games "sold well" (or something) they will add new content
I for once want to play as the generation 4/6 eeveelutions and other non pokemon from kanto and i hope this will be the case
I'd honestly rather see a Pokemon Let's Go for Gold and Silver than having a simple update that added in the later generation Pokemon. The main reason why is because it would break the balance of the game IMO unless they updated every trainer in the game to use the newer Pokemon they might as well make a new game at that point. While it would be cool, it would just seem strange, but then again we live in a world where Ultra Beasts are a thing now so I guess anything's possible.