Nice concept, but it already is taking shape somewhere ! What you are suggesting is a hypersonic stealth UAV with a capability to fire AAA missiles at incoming aircrafts. That's a new platform altogether, how can convert a missile into an UAV ? Its more like a stealth Predator using a ramjet engine is what you are talking about. The thing is a UAV with ramjet will have very low fuel storage, and the ramjet will consume fuel like anything even while in the boost phase. Secondly, I don't think it can engage a target at hypersonic speed, it will have to slow down to do that, which will make it vulnerable for a brief moment as the hostile aircraft can lock on it in that instant. Rather, the feature you are discussing is what the 6th generations fighter are meant to do in the future !