I'd like to make a species that was originally developed to be biomechanoid superwarriors by another empire, before it collapsed and its satellite states went to war or became otherwise isolated. Thousands of years later, those nations fell in to darkness or became real nations in their own right. My species has survived on a single plant; a barren, industrial wasteland covered in radiation storms, massive mountains, and rivers of molten glass. These creatures would be ridiculously tough and good at killing people, but possessing the intelligence and will of children, basically. They play around in the ruins of their progenitor's spaceships and fortifications and form small tribal groups, hunting down the monstrous, mutated abominations and killer robots that stalk the wastescape (The species would be able to survive on organic matter, or be able to break down ferrous materials for fuel).

I'd either join the interstellar community when somebody else discovers me (And they either willingly or unwillingly take me offworld) or the species accidentally starts up one of their ancestor's more stable ships and starts wandering around.