In this case I am inclined to say you're fine. A memory of Yang having shown up, Yang being distantly present and an awareness that she's there is fine within the context of the vision. Maybe photos of her around, or in someone's wallet. Maybe a passing wave as she goes off to punch something.

Because things will be moving into a more open sort of sandbox, the fact that "It is always possible" is something that can be played with here. Yang could show up, but there is no guarantee that she will. But you're welcome to tease that she could be in Trench in this vision.

This also applies to anyone else who would wish to see a castmate in the setting within the vision. They can't really be played in detail, but an awareness of their presence is fine. (The only caveat here is that this should not be a specific Yang played by a specific other player, as that wouldn't be fair. It's simply the fact that yes, a castmate could show up theoretically.)