I'd imagine the game being really laggy if they have each gen except x y or bw2 lol.
We already have jotho pokemons and they'll probably add sinnoh too since we have Bidoof.
Yes, Bidoof is part of April's fool. I think they'll still add sinnoh pokemons as in the same system as jotho pokemons
Unova would really do great, no one has ever pulled unova region in a poke-mmo game before or maybe someone did but I just didn't know about it?.
Aside from difficulty rendering stuff or whatever they do, I still think it has something to do with attracting new players in.