New Megas could help expand the type representation. Dark, Steel, Dragon, Fairy and Ghost are all very under represented. Including all existing Kanto Megas and Alolan Forms this is the spread for those type:
Dragon: 5 (Dratini, Dragonair, Dragonite, Alolan Exeggutor and Mega Charizard X)
Dark: 7 (Alolan Rattata, Alolan Raticate, Alolan Meowth, Alolan Persian, Alolan Grimer, Alolan Muk and Mega Gyarados)
Steel: 6 (Magnemite, Magneton, Alolan Sandshrew, Alolan Sandslash, Alolan Diglett, Alolan Dugtrio)
Ghost: 5 (Gastly, Haunter, Gengar, Mega Gengar and Alolan Marowak)
Fairy: 6 (Celfairy, Clefable, Jigglypuff, Wigglytuff, Mr. Mime, Alolan Ninetales)
In addition of existing Kanto Pokemon with Mega Evolutions the following types aren't represented at all: Electric, Ice, Fairy, Ground and Steel.
Now that Megas are confirmed to be in the game I really can't see them leaving 5 types completely unrepresented.
It may be a pipe dream, but I think there will be new Megas, which would absolutely be a reason to buy the game for hardcore fans who are wary of the "appealing to casuals/ Pokemon Go players" direction the games seem to be taking.