In my mind, ME:A will be:
Synthesize Ending: We won, organics and synthetics are now one united citizen of Milky Way. As our old problems eradicated, new problems emerged. People are growing, and fast. Post-war baby boom increase the population of the galaxy up to 500% and it's keep increasing as no one really died out, and finally after a few century, all planet in Milky Way is cataloged and colonized. No more room to expand. This sparks the idea of expanding to the nearest Galaxy that we never know before, Andromeda. So all bright minds and resources were gathered to create an exploration ship, twice the size of the Citadel. Then we selected the best people from across the Galaxy for the mission to seek new place to settle. After everything is ready, they activate the Citadel Relay, sending the ship halfway across into the Andromeda. And that's why we see the ship on N7 Day trailer heading towards the Andromeda, halfway already, because the Reapers create a network to the dark space in between the Galaxies. (The Goals is to expand)
Destroy Ending: As the reaper advanced, we don't have any choice but to escape. There are many people who've lose their hopes and propose an Idea about escaping to the Andromeda galaxy. In between creating the Crucible, people were also creating an ark to escape the Reapers, so if the reaper did overwhelm the Galaxy, our races won't be eradicated and we hoped one day that they will come back home. The Ark is a top-secret project that only a handful of people know it. The communication is severed after the Crucible fires, cutting all connections with the Ark, that they don't have any updates about the circumstances that they keep going to the Andromeda. (The Goals is to survive)
Control Ending: As the reaper threat eradicated, and now they're even helping us, we could achieve a lot of thing in just a short time. That make us curious about what is in Andromeda, and with the help of the Reapers, we could build a scout ship that carries large populations in order to establish connection with Andromeda. The Reaper construct a relay halfway towards the galaxy and we will go ahead and see if there's any advanced civilization that we could establish contact with, while the reaper will keep constructing connection from Milky Way to Andromeda. (The Goals is to connect)
That's what I think should've happened. It's just a mere theory written in bad English. Just let's hope that there won't be any shitty unbeatable enemies like the Reapers.