I've been wishing for the writers to find a way to bring Gallifrey back into the story. In the original series run, episodes involving Time Lord society were always among my favorites, in part because the Doctor was up against his equals; now, his only legitimate challenges involve facing down entire races. In the modern series, the arc involving the Master are also standouts for similar reasons. We also got a glimpse of the alien side of the Time Lords, something the Doctor rarely reveals.
I'd like to see the Rani brought back, too.