Gellar is playing the Seventh Sister, the masked Inquisitor we saw in the trailer. My theory? She's an Echani.
The term Echani was re-canonized in Star Wars Aftermath, although both the species and the martial art of the same name have largely been lost to Legends. (They're still marked as Legends on Wookieepedia.) In Knights of the Old Republic the Echani species was introduced as a species of clannish humanoid warriors. They had a highly developed body language and could read people by just their gestures. The ultimate form of expression for the Echani was fighting, and they felt that one couldn't truly know a person except by engaging them in combat.
Although the species hasn't appeared anywhere else in the new canon, the Echani martial art was recanonized in Aftermath. It's confirmed that the fighting style is well-known in, or even embedded in, the Empire.
...she's practiced in self-defense, having trained in the Imperial martial arts: a combination of Zavat, echani, and good old-fashioned ICE - Imperial Combat Exercises, the same training that every stormtrooper and officer gets.
So why does this fit the Seventh Sister? She has pale skin and yellow eyes, but both could be a product of dark side corruption rather than natural coloration. Gellar focused on her mannerisms in the video, and while it is far from unusual for artists to draw inspiration from voice actors' movements, that could indicate that the character is particularly expressive. From the clips we've seen so far, she has a distinct, quick fighting style.
Perhaps Darth Sidious and Darth Vader wanted to draw from a warrior tradition without direct ties to the Jedi. It's what they did with the Dathomirian Zabraks, after all.
Lastly, there's that name. The Echani ruled a star system named the Six Sisters, and if the Seventh Sister hails from there, maybe she sees the dark side as an extension of the Echani holdings, or her work with the Empire as something she needs to do for her people. Between the little mention of the Echani in Aftermath and what we know about the Seventh Sister so far, I'm beginning to wonder if Gellar's character is going to recanonize the species.
Update: It has been brought to my attention that Freddie Prinze Jr. knows the Seventh Sister's species to be Mirialan. Not quite official confirmation, but that's a thing.