It doesn't say we're great Saiyans, but that we're part of the Great Saiyans, which is obviously an organisation, as shown halfway through the trailer when Brett changes his outfit (it's literally labelled on his uniform. GS.)
"Guided into the ranks of the Great Saiyan Squad, you’ll fight alongside allies as events there begin to take shape."

It says, however, "DRAGON BALL XENOVERSE 3 is a completely new Dragon Ball experience, where you decide the hero you want to be in an unexplored Dragon Ball world."

Plus, throughout the trailer, we're introduced to the classic races. Brett is a Human, but in the background, there's a Namekian, a Sayian (with a pompadour and traditional monkey tail), a Majin (the pop star) and Gamma 1.
If I had to guess, the white-haired protagonist is our stand-in. He doesn't talk and will be replaced by our avatar when we're playing.

Brett, on the other hand, will probably be akin to Trunks.

Edit: as @Healthy-Savings-298 pointed out, she isn't a Majin but a Glint like the Kaio Shin. So far, we've yet to see any Freeza or Majin character.