Given that:
1) The game goes so far as to relocate the action to another galaxy.
2) The devs have made multiple statements that "we respect player choice" and "we aren't going to establish a canon ending."
3) The game is being made for new consoles, so save imports would be a gigantic pain to implement anyway.
4) Despite apparently being set long after the events of ME1-3, all of the technology seen in the trailer seems to be pretty close to what we saw in that trilogy (with the possible exception of the ludicrously fast FTL travel but that could just be creative license for the trailer).
5) The teaser seems to really be pushing a frontier theme, what with the choice of music and everything.
I'm willing to go out on a limb and make a few predictions about the game's backstory.
1) The game will be about migrants from the Milky Way galaxy, or the descendants of those migrants, who left the Milky Way for Andromeda either before or early on in the events of ME3.
Why that particular point in time? Simple: it's obvious that the main reason to set the game in a different galaxy is so they can avoid bringing up the events of the original trilogy and thus invalidate player choices. But the choices of ME3 are really hard to ignore no matter how far in the future and how far away the game is set. Obviously there's the huge can of worms that is the ending, but even before then there are big problems. For example, we know from the trailer that at least one krogan made it to Andromeda, so Shepard curing or not curing the genophage would have pretty big impacts on this setting. Unless, of course that krogan or his ancestors had already left before those events took place.
Having everyone leave the Milky Way before any of the big decisions of ME3 makes things much easier for the writers to deal with, as all of the ME1 and ME2 decisions were either small scale (whether certain characters live or die) or just things that the denizens of Andromeda wouldn't care about (like who runs the Council) and thus would have no reason to mention. Though maybe there will be a save import feature that will let old time fans hear characters tell tales about the deeds of their Shepard.
2) These migrants fled the Milky Way in order to escape the Reaper invasion.
What could possibly motivate a bunch of people to pack up and leave for another galaxy and never return? Maybe because if they stay in the Milky Way there is a high likelihood that they will be killed by giant robot space squid.
3) They traveled to Andromeda through some kind of portal, either naturally occurring (IE a wormhole) or artificial (IE a super duper mass relay) that people stumbled on.
Since the point of departure is probably going to be before or during ME3, there is no time for the Milky Way races to develop technology capable of intergalactic travel. Near lightspeed sleeper ships are a possibility, but I think that far more likely is that people will just find some kind of portal to Andromeda around that time. Some may call this contrived, but that boat sailed a long time ago.
4) The portal was closed after the refugees went through.
If the Milky Way and Andromeda remained connected, that would defeat the purpose of moving the action to another galaxy. So this has to be a one way and one time only trip. Most likely, the portal is closed to prevent the Reapers from following. This leaves the refugees 2.5 million light years from where they came from with no backup or support, and they have to search for a new home.
An old cowboy went ridin out one dark and windy day
Upon a ridge he rested as he went along his way
When all at once a mighty herd of red-eyed cows he saw
Plowin through the ragid skies and up a cloudy draw
Their brands were still on fire and their hooves were made of steel
Their horns were black and shiny and their hot breath he could feel
A bolt of fear went through him as they thundered through the sky
For he saw the riders comin hard and he heard their mournful cries
Retour en haut
