Before I start, I would just like to preface this by saying that I will not reveal how I have gotten this information so that It cannot be traced back to me.

There will be new forms you can activate through the Z-bracelet for certain Pokemon who have experienced many battles.

This is the secret that the starters and Rockruff share.

Some examples are Greninja, Lickylicky, Mamoswine, Eelektross, Ursaring and Haxorus.

There will be 45 Pokemon Receiving Alola forms.

Some examples are the Geodude line (Steel/Fire), Bellsprout line (Grass/Dark) Ponyta line (Ghost.Poison), Staryu line (Water/Steel), Goldeen line (Fairy/Ghost) and the Voltorb line (Water/Flying).

Only generation I Pokemon lines will be eligible to receive these new forms. Pokemon such as Oddish who have non-generation I Pokemon in their lines cannot receive these forms.

Megas will be returning, and will be announced at worlds along with Mega Dragonite (Dragon) and Mega Crobat (Poison/Flying).

Other Pokemon to receive megas will be Bellosom (Grass/Electric), Flygon (Dragon/Ground), Scrafty (Dark/Fighting), Goodra (Dragon/Water) and Ambipom (Normal/Fighting).

You will only be able to find the mega stones during the post-game. You can only find the newer mega stones scattered around the region. Older mega stones can be bought with BP and acquired through other means.

Team Skull unknowingly summons either Solgaleo or Lunala by capturing the Island Guardians and disrupting the natural balance. This ties in to how Zygarde is introduced.

Lillie is indeed in possession of a Zygarde core.

HM's will not be returning, instead, ride pokemon will take their place.

The Pokemon you can ride are Charizard, Wailmer, Sharpedo, Lapras, Whiscash, Mantine, Mudbray, Mudsdale, Tauros, Stoutland, Avalugg Hippodown and Luxray.

You will not be able to visit other regions, but the game does offer a rich post-game.

There will be a total of 97 new pokemon.

Marshadow will be Psychic/Poison.