Welp, I have a couple theories.
Given that the last two main protagonists were introduced with that same turn around and draw gun hero shot (Shep and Ryder), I think the teaser trailer was showing the main character of the new game.
Who is that main character?
We know Liara is a player in this new game. We also know that, regardless of the choices Shepard made in terms of romance, he "embraced eternity" with Liara multiple times in the original trilogy.
I suspect that the new Player Character is in fact the daughter of Liara and Shepard. Liara, in her role as the Shadow Broker, has gotten wind of some new threat, and figured it was time to bring another Shepard into the galaxy.
It's all circumstantial evidence but a couple things I picked up on: The new character is an N7 operative. That would be very on-point for the child of Shepard. But...they're also sporting stealthy gear and a suppressed pistol....very much like you might expect of an agent of the Shadow Broker. I think she's both. If so....it's possible stealth gameplay may finally be coming to Mass Effect....huge win IMO, even if it's only for certain levels.
And having an Asari graduate from the N7 program isn't really far fetched. Cora, in Andromeda, was allowed to train as an elite Asari Huntress, the Asari equivalent of N7. If this new character is in fact the Asari daughter of Shepard, that Shep pedigree would explain why she might be allowed to do the same.
In terms of development, it would also help simplify things for the dev team. Since all asari are "female" they'd only need one voice actress this time around (Katee Sackoff please?), and they wouldn't have to double record any character lines from other people. And it would still leave all the romance options available and they'd all make sense in-universe, more-so than the previous games.
Anyways, that's my theory.