I only see one problem with this: The kinetic energy of what equates to a 600-3000 ton object hurtling towards the surface at 40-50% light speed would likely be enough to shatter the entire planet/construct/whatever that the map is on. That energy would be equivalent to almost 10 tera-tons (10,000 megatons TNT) Yeah, I know Halo wars did it, but it was significantly modified to fit in with an RTS, like most everything else in that game. You could probably get away with it if you just dropped something from orbit, in that case, it would only flatten everything in a 10 mile radius of impact. Other than that, it seems OK.