I honestly don't see a way for the Reapers to actually come back. The corpses strewn around in the teaser trailer are, imo, an indication that the Reapers are gone for good, and that there's some new threat that's central to the game's story.
The other teasers make it seem like something to do with "defiant humans", which, if done correctly, could work well in both cases where Shepard is brought back or stays away (dead from natural causes or otherwise).
This is pure speculation, but I could see said "defiant humans" as a group of fanatics who have turned Shepard (either alive in the Andromeda galaxy thanks to the "Ark Six" mentioned in the first teaser, or just simply dead) into a messianic figure ("The Shepard" from the ME3 post-credits scene), and they are defying the Council by creating a mass relay that links Milky Way and Andromeda together.
I could see these humans as one of the antagonists of the next game, though I suspect that the Kett and the Remnant (Jardaan) will play a bigger part, regardless of Andromeda's shortcomings.