[Baltan II]

My guess based on what FRLG and HGSS did is that the Hoenn Dex will remain entirely intact and unchanged. Unlike those games, the Dex doesn't have to be adjusted for move evolutions because no Pokémon with that evolution style is native to that region and item-based evolutions, locations or Incenses that were made in Gen IV and later could easily be excluded until the postgame content.

I wonder when the game will allow for the use of Mega Stones, though I'm guessing postgame too. Would be interesting whether the Gen I starter stones, Abomasite, Aerodactylite, Ampharosite, Garchompite, Houndoominite, Kangaskhanite, Lucarionite, Mewtwonite X and Y, Scizorite, and Tyranitarite remain exclusive to XY. Interesting that many of the stones I've named are available during the initial story of the game and don't require the postgame sundial quest to obtain, like they were intentionally made easier to obtain in preparation for just such a remake.