River Song, Merlin, and Battlefield

Philip Sandifer discusses Battlefield:

Yes, we’ll probably never see the Merlin period of the Doctor’s life explicitly (although the novel’s assertion that the Merlin incarnation of the Doctor has red hair provides humorous justification for the Doctor’s desire to be ginger).

Which is a pity in some ways, as a storyline rooted in the ancient heritage of Britain about a mysterious woman who rises from a lake, who eventually traps the Doctor in a prison for all eternity, and to whom the Doctor appears to live backwards in time would be really, really interesting.

Spoilers?