4. It's Time For An Older Actor (Wait! Hear Me Out!)
Steven Moffat originally intended to cast an actor in his forties or late thirties in the role, believing that too young a Doctor would just be a sex symbol or a stand-in for David Tennant, and that too old a Doctor would be worn out by all the rushing about (see Hartnell). In his own words, "Of course I ended up casting a 26-year-old." No matter what you think of Moffat's casting (if you believe it, there was a rumour that the Moff only cast Karen Gillan because she was hotter than the girl he auditioned before her) I wholeheartedly agree with his judgement on the Doctor's age. Christopher Ecclestone (who up until recently was MY Doctor) handed in a great performance as the "grand old man of the Universe" and did it without needing to be a handsome young hero. Tom Baker, considered to be the best Doctor before Tennant, was cast at the age of 40. The theory holds up. Izzard is currently 51 so it's time to grab him before he runs out of breath. Should he be able to balance intellect with physical stamina, we could be looking at another great in the role.