Eh. I know I am in a tiny minority here on GAF, but I think it was a masterstroke only including the Gen 1 Pokemon and a huge part of the game's appeal for me. I know for people who have stayed close to the franchise for the entirety of the last two decades it probably feels shallow and redundant to be catching this fraction of the total monsters yet again, but for me it's a thorough nostalgia-gasm as well as a very strong rekindling of that initial sense of potential accomplishment- that I could actually feasibly collect all 151.
I'll refrain completely any discussion of the actual quality of the Pokemon beyond Gen 1, not referring to that at all right here. But just the fact that there are more, and then more after that, makes the prospect of "catching em all" more daunting and less practical for me. Right now I am primarily excited about one day having "all" 151 Pokemon in my collection. If more are added before I've had the chance to even do that, it'll actually make things a lot less interesting for me. That's not even getting into the possibility of trying to keep up with/collect all... what are we up to now, 650?
Basically there are a lot of different kinds of players for a game like this. That fan concept for introducing Gen 2 was actually the first concept I'd seen that might actually make it enticing for someone like me. I can completely understand why a majority of people posting on this board would prefer that many more Pokemon are added, as much and as often as possible, but I did want to offer my perspective.
The only new Pokemon on the roster I actively want (i.e. would be completely okay with being added to the game 5 minutes from now) are the pre-evolutions and further evolvutions of the Gen 1 mons. People like me know Pichu and Scizor from Smash Bros and have seen that there are other cool evolutions for Eevee we never got to play with. At this moment I'm totally down for those. But the further we go into expanding the roster, the less appealing the entire proposition is to me personally. It's a tricky balance for Niantic.