I think it would be cool to give each generation its own Pokemon Origins but the scenarios you bring up are all pretty much the same. The first Johto one you suggest is a direct mirror (at least in my opinion) of Pokemon Origins with Ethan battling the final legendary of the game with his starter Mega evolving. I understand that it was a neat surprise in Origins, but if they were to do another one in Johto, I'd like them to deviate from the first plot. Instead of filling the Pokedex, perhaps Ethan wants to be more like Green and really just go after badges and being powerful. Afterall, although the original goal of your character in the game is to fill the Pokedex, X&Y have shown all the different paths that a trainer can take. If they were to do it, I don't want a rehash of Pokemon Origins. And, although part of the game, I don't think Ethan going to Kanto would be necessary. We already got an adventure in Kanto, end the final episode with Oak inviting Ethan to Kanto and end the special (just like Red realizing that Mew exists and wants to find it).