The Age
If you were to ever conform the Doctor's character to an archetype of classic literature, the Doctor would be the Wizard, the wise old mentor, the outsider looking upon the world, doing what he can to help but ultimately isn't the one the audience should identify with personally. Which is why an easy way to get this idea across is to find an older actor right? But aren't the Doctor's getting younger? Solution, we need an older Doctor! The Doctor is a character with such a long unwavering history and whoever they cast needs to portray this element too. Matt Smith was a rightful exception to the rule as he was 100% capable of portraying the age of the character in spite of his youth, in fact I dare anyone to watch the scene in The Wedding of River Song where he learns of the Brigadier's death and tell me other wise. But the Doctor as a character needs something to physically age him at this stage, something to ground the character to a form of reality, a regeneration into an older incarnation should do the trick. Another reason for this being that I'm sick of the Doctor being seen as a sex symbol, but that's a story for another day.