Well, given it is said to be a faithful remake ... duh.
If the rumors of Mega Evolution returning, it appears that we could have every Gen I to IV pokemon in the game (as Mega Evolution from Gen V and VI total only 2 pokemon.) We may ever get select pokemon from later gens (Sylveon, the new Regis, and possibly the Galarian Form evolutions).
We really know very little about these remakes right now outside the statement they are a faithful remake AND they appear to have the extended dex from Platinum at the very least.