I've always liked the idea of humans being the underdog, winning only through chance and home field advantage. I'm also a big fan of showing the dysfunction of government and military; for what I mean, look into
Quagmire at Mapster. (The author goes all-in on bad intel, officers pursuing their careers over goals, politicians throwing away lives for a little extra money, and flagging public opinion. The campaign is basically Space Vietnam, I love it.) Under prolonged stress, society would naturally fall apart, however, and we can't just have a re-hash of Battlestar Galactica, with refugees living in space, but it could gain some inspiration from that.
Of course, constant warfare would reduce humanity to nothing. Fortunately, most wars aren't constant; there's an ebb and flow. I think humanity could survive, living under the shadow of the greater Protoss and Zerg conflict.
Another thing I love about Quagmire is that it doesn't undersell tragedy. In StarCraft, so many worlds get destroyed, and nobody bats an eye. I'm like, what? An entire
world has been destroyed, the culture and history unique to it
ended. The
only time I feel there's any respect to accorded to the Protoss or Zerg or the earth-shattering chaos is when Jim Raynor scoffs at sending Kerrigan alone to New Gettysburg; you can hear the incredulousness in Clotworthy's voice: "First you sell out every person on this world to the Zerg, then you ask us to go up against the Protoss, and you're gonna send Kerrigan down there with no backup!?" In Quagmire, a colony is nuked out of existence, and the author totally nails the horror and tragedy and senslessness of it all.
I want
that in my StarCraft.