Try a three-dimensional crosshair. Use a sort of translucent wireframe box (not actually wireframe, but use narrow boxes to form the edges) to make a sort of futuristic space crosshair.

I've used a 3rd person crosshair before, though not in a space shooter. I made a helicopter game once that had two views onscreen. One was a view behind the helicopter, and took up most of the screen. In the corner a smaller window had the view from right next to the helicopter's gatling gun. I used c_trace to determine the distance ahead of the gun and used vec_to_screen to place the crosshair over the object.