The part about not wanting to include an active sulfur mine in a game about protecting the environment is understandable, but if they do end up making blue Slugma/Magcargo I wouldn't be too upset. It'd give them something to be known for besides being OHKO'd by water-types and, most importantly, I like snails and slugs.
I completed it in gens 6 and 7. Gen 6 made completing the national Pokedex a very accessible goal, almost all Pokemon are catchable between XY and ORAS, you were able to search Pokemon you've never seen on the GTS, and you had wonder trades, Bank and ORAS Blissey bases.
I even bred 3 living dexes, two for gens 6 and 7, and a spare to transfer to gen 8. I never bought SwSh, so now I've got 600+ Pokemon wasting space in my PC boxes.