Manned Spaceflight, perhaps using the dummies from SimplePhysics or Dummy Defense as the astronauts?
As Felipe Falange from Squad once said, "Kerbals make KSP more than just a game about spaceships." Right now, SR is just a game about spaceships.
So we need some astronauts to make us care more about rockets. Right now, SR is not very emotionally investing. Having astronauts in the command pod would make me care a lot more about playing the game, and would really improve the whole experience.