Easiest question in the world. I don't even have to get creative. All I have to do is pull up content they had and cut.
Mass Effect writer Drew Karpyshyn revealed his original intent for the ending of Mass Effect 3, which involved the concept of Dark Energy, which was hinted at in the previous two games, such as on Haelstrom during Tali's recruitment mission, when she commented that the planet's sun was an advanced age for no scientific reason.
The Dark Energy was a force that was going to consume everything. According to Karpyshyn, "The Reapers as a whole were 'nations' of people who had fused together in the most horrific way possible to help find a way to stop the spread of the Dark Energy. The real reason for the Human Reaper was supposed to be the Reapers saving throw because they had run out of time. Humanity in Mass Effect is supposedly unique because of its genetic diversity and represented the universe's best chance at stopping Dark Energy's spread."
The original choice was between killing the Reapers and trying to find a way to stop the Dark Energy threat with what little time was left before it consumed the galaxy, or, "Sacrifice humanity, allowing them to be horrifically processed in hopes that the end result will justify the means."
That. With an epilogue (dramatized, preferably, not just text on screen) detailing what happened to all the surviving characters and give us glimpses of the different races and places we've seen. Since both choices leave lots. Just no humans in the second.
And the two hour Earth segment leading up to it would have played less like a corridor shooting gallery, and more like the Suicide Mission from ME2, with all of your decisions, esp. ones regarding squaddies, coming into play for the most dramatic pay off in the series.