Here's what I have for you guys:
It's a surprise but here's what Imma tell ya. The timeline is vague on how many avatars past etc will be slowly revealed but tech wise it's 21st century.

The world enters a new era to where Zaheer's ideals put to reality with some tweaking of course, it is a world where there is no Avatar and while there are Fire Lord, President of Republic city and all leaders. All of them were ruled under one Emperor, a rule of the Dark Avatar. Imma call Book 1: Restoration.