Cartmel never got to write an episode during his tenure as script editor, as he instead simply created a masterplan to radically change the Doctor's character. Its like being a gang leader who has everybody else do his murders for him. However, his expanded media work has been pretty hoof, so evidently he can get his hands dirty fairly well when necessary. Considering how good he was at making Sylvester McCoy - the world record holder for stuffing ferrets down his trousers - into one of the darkest iterations of the character, he would be a perfect fit for the Twelfth Doctor. He even managed to coax some life out of Nyssa in his Big Finish play, too, so there's even some hope he could bring interest to the human glass of water herself, Clara Oswald.