It's an indication that the Doctor hasn't been Merlin yet. Per "Battlefield," the Merlin Doctor has red hair. So, Tennant's Doctor saying in "The Christmas Invasion" that he wanted to be "ginger" means that he's not yet the Merlin incarnation. It was a sneaky, subtle continuity ref back to the old serieshave any of the previous Doctors, ever wanted to be ginger, or was that just Ten?
Where did RTD state this? Or is this just your "theory"?