https://pastebin.com/4LahdrCA
Confirmed, the only new megas in the base game are the ones we have know for months.
>Clefable
>Victreebel
>Starmie
>Dragonite
>Meganium
>Feraligatr
>Skarmory
>Froslass
>Emboar
>Excadrill
>Scolipede
>Scrafty
>Eelektross
>Chandelure
>Pyroar
>Floette
>Malamar
>Barbaracle
>Dragalge
>Hawlucha
>Zygarde
>Drampa
>Falinks
>Chesnaught
>Delphox
>Greninja
While Heatran, Darkrai and Zeraora have associated megastones, their mega forms don't exist, so that's pretty much guaranteed to be Mega Dimension Rush content like Mega Raichu.
Also the list confirms the following megas are not in the base game either
>Sceptile
>Blaziken
>Swampert
>Latias
>Latios
>Rayquaza