This is one guy’s opinion. One guy’s opinion based on looking at the trailers. I don’t pretend to have been keeping up with all the latest gossip or what-have-you and I hasten to add I’m not much of a gamer. But here goes:
Your character is part of a billion+ person, multi-species mission to seed our neighbouring galaxy, Andromeda, as a plan B during the Reaper War in case the Reapers won and wiped out all life in the galaxy. This mission is achieved by putting everyone in suspended animation in a citadel-like ship (see teaser trailer) and sending this vast space ark off into the big black.
You wake up what seems like moments later but is actually thousands of years and set about constructing mass relays and cities (see growing city in trailer). Fights break out etc, which your character has to settle (possibly in a shed load of repetitive side missions. But let’s hope not).
This is where things get epic and complex:
You see, thanks to Shepherd, the Reapers didn’t wipe out life in our galaxy. Unprecedented, that. Reapers always wipe intelligent life out. They didn’t get the name ‘Reapers’ selling artisanal red velvet muffins after all. But what happens if they don’t reap? What happens to their untended sentient ‘crop’ (i.e.: us)? What terrifying chimera of bio and cybernetic life emerges?
I’ll tell you what: MASS EFFECT: ANDROMEDA’S BIG BAD!!!
It’s had thousands of years to develop and perfect whereas you, dear colonial, have only just stepped out of your cryo-pod full of naive new frontier dreams.
The BIG BAD has developed relay tech that can cross galaxies. The rest of the ME:A trilogy, then, is spent with you fighting this wave of sheer, nightmarish Hell-Ass and secretly coming round to the Reapers’ point of view.
Aaaand… that’s my theory. Such as it is. An intergalactic shitstorm plus associated DLC.
(Also, Bioware’s use of Ghost Riders In The Sky is a secret taunt. The BIG BAD will, metaphorically at least, resemble the ghost riders in that haunting country song.)
(Addendum: Outside chance BIG BAD will be voiced by Shepherd, whose become the reincarnated vocal locus. I say this because that would be the easiest way of shoehorning him/her in without dragging all the requisite background information from the last three games)