Late to the discussion party of 2020, but I have plenty of theories. All of this is assuming the teaser trailer provides plot details and that things were foreshadowed in 3.

Location is Milky Way, several centuries in the future, destroy ending canon. There is just no way around it frankly, synthesis is ridiculous and control ending just has too many implications. The ridiculously destructive nature of the color Red provides the most possibilities for the writers to write themselves out of the corners. Decisions just become forgotten, misconstrued or irrelevant. I really want it, but I don't think full import will be possible. Best case scenario a bunch of parameters, like chosen ending, are outright ignored.

Shepard might survive but will be long dead of old age by the game's start. So will all other companions excluding Liara. That way, she can retell to the player what happened (decisions) during original trilogy and how these effect or do not effect the new situation in the galaxy.

Humans, turians, salarians and asari are still powerful but in severely weakened state. Probably too busy rebuilding to curb the rising issues in the galaxy. Descendants of Cerberus enclaves still a threat and being hunted down.

Rannoch decision sidestepped entirely. I am leaning towards Quarians surviving either way, but just barely (assuming some survivors at Rannoch). Geth likely wiped out, but small enclaves may have survived the blast or Rannoch.

Krogan likely a big threat. Some say Krogan homeworld is destroyed but that's irrelevant at the end of the day. It's not the only Krogan dominated colony. Without interference, Krogan are far more likely to develop the cure themselves or just develop resistence. How they were treated will probably impact resolution this time around.

I hope Rachni don't reappear, I can only make them extinct so many times.

The big obvious new race is Yagh. It's foreshadowed in 3. They are too primitive in the current cycle so they escape harvesting. With galaxy in state of ruin, they are in prime position to become its next rulers. They just need to study the wrecks of the council races and they have the necessary technology.