If we assume story reasons for promos and no doubling on promos we can make a list of likely canidates.
Vilains who already have promos
Blade
Omnitron
Gloom weaver
Spite
Villains whose story is already being advanced by Villains
Miss Information
Plague Rat
Ambuscade
La Capitan
Villains who are new enough a promo already seems unliley
Kaagra
Infinitor
Wager Master
Progeny
Deadline
This leaves us with
Akash'Bhuta
Kismet
Apostate
Voss
Dreamer
Dawn
Ennead
Chairman
Matriarch
Iron Legacy
Vengeful Five
Vengeful Five is highly unlikely, their place in the story is established and unlikely to evolve (after all they were briught together by Blade who is out of commision until Tactics due to the side effects of his boosting serum). Chairman is unlikely, the organization is crushed and he disappears until Tactics. Dreamer is unlikely, mostly because poor vanessa long is already suffered a lot and her place in the story would likely be a one off deal. Same with Iron Legacy, The Ennead and Matriarch. Iron Legacy was dealt with and should not return, The Ennead cause Horus Ra who then came back and beat them, seems little ground for a resurgence that would alter how they play and Matriarch probably won't get a promo because her change is to become a hero in Tactics.
This brings us to Akash'Bhuta, Kismet, Apostate, Voss, and Dawn.
Voss would be awesome, coming back from the Realm of Discord to take Vengeance on Earth, but I feel that's going to be in Tactics because we've been told we won't recognize him and a promo doesn't change how a deck plays to that extreme, especially one as tight as Voss. (Tangent: Could Voss be the Mist taking people in Tactics, using them to build a new army before marching forth?)
Dawn would be good, but Hammer and Anvil appear in Villians so it's at best 50/50 in my mind.
Kismet could be, but I doubt it. She's a pain and a trickster, kind of like Joker. She keeps coming back but she doesn't change much overall.
So my money would go on Akash'Bhuta and Apostate. Akash we know changes by Tactics, those purple crystals everywhere and it's likely that started before Tactics. Also Akash'Bhuta can be a fun fight but I've never felt her flipping really meant anything and she is too easy in environments with little or no targets (fighting her in the Freedom Tower is almost boring). I think a promo to make her more active and a more devastating threat would be good. Apostate is almost the obvious one, we know there is a lot about his story that is unclear and needs to be revealed. A promo for him is almost a must to clear up what his deal actually is.