(sorry for post spamming)
I switch between IT and non-IT and Other interpretations on a fly now. Most of my posting above in this thread was non-IT, but my personal theorizing has things more to do with a sort of simulation going on, perhaps in a Reaper itself, and Synthesis is just the choice that illuminates how Matrix-like Shepard's situation actually is, as he fumbles in the dark and ignorance. Meaning that each choice could still affect things (considering the nature of 'virtual' in Mass Effect), but we'll be railroaded into a more specific galaxy situation in the next game regardless.
That's a cool theory. I had a similar idea if Dark Energy was ever an ending.
It involved exploring the Harvest ending, is a more Matrix style ending. It would be a concept that would still get you a happy ending where life goes on, but in a creepy dark way.
The idea would involve Shepard giving into the Catalyst's demands that Harvest is the best route. The Catalyst then tells Shepard that he must be harvested as well.
Before you get harvested, there are 2 options. Kill yourself before it happens, or surrender yourself. If you did, Shepard is harvested. And during the process, Shepard asks "Will I remember anything?" And the Catalyst would answer, "We can't allow it".
Then cut to a losing battle outside, and the Normandy is shot down, and crashes. Joker or your LI awakens in the wreckage, and sees Reapers landing near the Normandy, before passing out.
LI then wakes up, and sees the land covered with Dead Reapers. They then sees Shepard, which she/he embraces. LI says "We did it!", and Shepard says "Yes... And we're all alive".
We then dissolve, and see the fleet of Reapers leave the galaxy to Dark Space after finishing their harvest, and realize we're actually inside a Reaper. Proving to us that every Reaper really is a nation of lives, living lives.
Thus, if anyone made an ME4, it would be made with the assumption that we defeated the Reapers, but with a creepy possibility that everything you're playing from here on out is actually in a Reaper simulator.