Concept: Thoughts on a side story / post-game concept?

[Herokin64]

In my current fan game project, which I'm titling Pokemon Eternity, I plan on having a side story focused around the Legendary Quartet known as the Avatar Quartet. Basically, they each created and control a unique form of some Pokemon titled Avatar Forms (only available through this encounter). Also, due to the whole "Avatar" name reminding me of the show Avatar: The Last Airbender, I've decided to have these Avatar Forms be of Pokemon gaining one of four types, those being Fire to represent the Fire Nation, Water to represent the Water Nation, Ground to represent the Earth Nation, and Flying to represent the Air Nation. My only question as of now is this:

Should I have the Avatar Forms be of only Sedayan Fakemon (Sedaya is the Region Pokemon Eternity is based in), only old Pokemon, a mixture of both (two Sedayan, two old), or do I have them be a post-game story involving Formes of previous Legendary Pokemon?