BioWare once intended Mass Effect 3 to end differently than it did.
One possible ending concerned the spread of Dark Energy - a force used for mass effect fields and biotic powers. It goes that Reapers were created to stop the spread of Dark Energy, which would ultimately destroy everything. That's why Reapers, every 50,000 years or so, processed (turned into a Reaper) an entire species - to slow the spread of Dark Energy. The Human Reaper was to be the last throw of the dice for the Reapers. The ending of Mass Effect 3 would be you deciding whether to sacrifice the entire human race, and create a Human Reaper, or take your chances that humanity could come up with another alternative.
That ending, Drew Karpyshyn came up with. It was one of several, apparently.
In response to that being aired,
Drew Karpyshyn wrote a lengthy blog post on his website. "Yes, we had a plan, but it was very vague. We knew we wanted to focus on some key themes and bring in certain key elements: organics vs synthetics; the Reapers; the Mass Relays. Beyond that, we didn't go into detail because we knew it would change radically as the game continued to evolve," he explained.
