The prospect of adding Johto as a new region seems entirely logical to me for several reasons.
Going beyond the argument of nostalgia, the second generation pokemons originate from the Johto region of the Pokémon Silver game, which is the second best selling pokemon game of all time.
The iconic johto region offers us a great wealth of playability, in terms of scenario, good soundtracks (if not the best ones) which would make PokeMMO all the more attractive in terms of game content.