Broken Mass Relay
One of the first significant details we see in the trailer is actually pretty easy to miss, as you can write what we zoom by at the 45-second mark off as junk or a destroyed ship floating in space. Pausing the video shows that it is clearly a broken Mass Relay.
Anyone who knows Mass Effect knows the importance of Mass Relays to the series, as they are the transit devices responsible for faster-than-light space travel throughout the galaxy.
While Control and Synthesis endings leave open the possibility that the Mass Relays could be repaired, in the Destroy ending they are in disrepair without an obvious way to fix them.
While showing this and putting in a Reaper horn sound effect might seem like it could just be an homage to the original trilogy, everything points to the fact that things will pick up at some point in the not too distant future from when Mass Effect 3 ended.
