I keep imagining something green, but that is pretty cliché. I think drawing more toward the industrial electrical solutions for sustainability would be more in line with this sites identity than drawing on the mother earth nature side of sustainability.
So I am picturing solar panels and windmills electric cars, bicycles. Look at bicycles.SE and diy.SE. Some combination of those designs that kind of lends more toward the engineering/math aspects of sustainability.
For example a giant compost pile with windmills and solar panels on it, along with electric cars hooked up to it. And people planting gardens around the big pile.
We wouldn't want to alienate the Rural vs Urban visitors, so showing something with a city, or showing something with a farmhouse might do just that.
This is all just for discussion :).
As a logo for the question feeds on our chat, I've used an ouroboros.