I would really like to see a plot where characters have to work out a problem that doesn't necessarily involve combat. I'm thinking back specifically to the Slenderman plot, where it was less a matter of just handwaving some research in the library, which is what usually ends up happening, but actual physicals clues to find to and discuss and act on. Something that builds up tension over time (and this can cross over genres with action or horror very easily, but what I'm really looking at is some good old fashioned intrigue).

As a general side note, I do like general mingling plots every now and again. Stick's autumn masquerade ball, for example, was a whole lot of fun, and I'd love more of that kind of thing. But I find that I get really bored with sandboxes. It's just a personal taste, but I prefer some kind of hook to make my characters act or explore or what have you, though I understand from experience the time it takes to run a plot like that.