I have a strong feeling that we're not getting anyone other than Korrina.
Honestly not even sure if we'll get a new character unless they reveal them alongside the next information drop.
I mean unlike G-Max and Original Megas all new Megas in ZA (bar Kalos Starters) were either seen as Rogues or used by important NPC.
We know many of New Megas as Boss Battles but it's around half. The other might be used by some important trainers. Especially since we are missing Psychic, Grass, Ice, Rock and Flying Specialists in base game.
Main problem might be withe the Flying one unless some new Mega gets Flying typing (since Startaport is a Boss Battle) as we don't have more new Flying Megas.
We have Scovillain for Grass, Crabominable/Baxcalibur (less possible since it's Stone online reward) for Ice, Chimecho for Psychic, Glimmora for Rock, Golisopod for Bug.
I have a feeling Mimi goes rogue, evolves hence the Meowstic boss battle.
Tatsugiri is also a Boss battle.
I can personally see Golurk getting the Flying type when mega evolving, Maybe Ground/Flying since it's a solid typing and we saw Golurk flying.
We saw Golurk used by some trainer in the 1st Trailer so there's a chance that trainer Mega Evolves it and he is somehow important.
Interestingly, Magearna is not a boss fight so it might be a gift Pokemon or used by some trainer (or both). Or it's stone will be a special distribution like Diance and Mewtwo since those were not Rogue Mega Battles (sadly, especially with Mewtwo having 2 Mega forms)...