While an origin and motivation to the Reapers on paper sounds like a good idea in reality it isn't. Someone said something similar earlier, but basically sometimes there really is no good explanation for why someone (or in this case something) does something. There have been countless psychopaths throughout history that have done terrible things and we won't know why exactly they did what they did. Any answer they could give you would be nonsensical or too vague to give any real meaning.
That's why I didn't want an actual motivation to the Reapers. They are an ancient race of machines that have been exterminating advanced lifeforms periodically for roughly the past 1 billion years. What the hell possible motivation could they have other that they are insane and are doing it for the sake of doing it? I knew that any possible explanation of their motives just wouldn't make sense and/or would be contrived and this is exactly what we get in the post EC endings. They just aren't the kind of antagonist that needs an actual motivation. They aren't humans or a similar analogue. They are billion year old machine/organic hybrids of some kind. They are supposed to be on a higher plane of existence. They have existed longer than some stars have. Their perspective is so completely beyond anything we humans have. Their motivation really should have stayed as "...not a thing you can comprehend." It's not a cop-out if they don't have a motivation: it would be simply staying true to what the Reapers are supposed to be.
An actual origin however would be interesting. While it isn't necessary for me to enjoy the story for the Reapers to have an origin, I would be interested to see what Bioware could come up with. Right now the origin is a little to vague, maybe Leviathan can clear some things up.
I really don't know though, Bioware seems stuck on keeping the Catalyst and the contrived ending choices, which is exactly what I think is wrong with the endings. I don't know how "greater context" is going to actually fix those problems.
Also, about the picture Nightwriter posted, yeah my pyramid would basically be an upside down version of yours. There are plenty of narratives with great characters and settings, but few in my opinion with a consistent and logical plot with important details explained. Good characters and an interesting setting are what help me get into the story, but if it's done at the cost of a consistent and logical plot then I really don't see why I should care about that story anymore. It's why I'm so frustrated with the ME3 endings.