I don't know who
I want the 12th Doctor to be (not in terms of actor, in terms of personality and behavior), but I do know what I would like narratively.
I want closure to the Moffat era. It's likely to be his final season in the center seat, and I want questions like "Who blew up the TARDIS" answered. I don't want an overarching storyline about the companion. I don't want timey-wimey shenanigans. I want the Doctor to face the problems of his past fresh and put them behind him so that, under the aegis of a new producer, the next Doctor can start his second year free of the detritus of the Moffat era.