I actually never really thought seriously about the possibility of Charles's geass. However, now that I read Buffalobiian's post, one thing occurred to me: What exactly is the geass code? And why exactly does it keep its host immortal? If we look at this whole thing from the perspective of the geass code itself, and think it's some sort of a parasite or infection wanting to survive, then it would be natural it would leave the annihilating Charles's body and jump to someone near, and if an existing geass power is a prerequisite, then it would have had to be Lelouch in the World of C, even though it was another code that gave Lelouch's power.
Yeah, if we consider the geass code something similar to One Ring, granting strange powers and giving an unnaturally long life yet having an agenda of its own, and certainly wanting to survive, a few things would be explained.