Man, I love the BBC! If I could go back in time (are you out there, Doctor?), I'd do everything in my power to help the Red Coats win so we could get to see this stuff sooner in the colonies!
Not so bad actually. There are real legends about Merlin as a young man. He was supposedly half-demon. Of course, it could be cheesy if they try to update him as a ASBO hoodie-wearer in the dark ages or some such nonsense, but there is a great deal of space for interesting stories in his youth. He could be played as quite a dark character, although cynically I suspect it is more likely he will be played as a Harry Potter rip-off.