Okay but changelingverse!who. But changelingverse!rosedoctor. Where Rose is a sweet little songbird, like a canary, and she flits around the control room when she has to exercise her shift. And the TARDIS translates shift speak of course so the doctor has no trouble chatting with shifted changelings.
And Rose talking with nine, and doing dumb dances, his head bobbing and her head bobbing to loud nineties music he puts on cause he knows she loves it. Her sitting on his shoulder but tumbling around when she tries to land on his head.
So he regenerates with the fullest, floppiest, best hair. And Rose lands in it and plays with it while he works on the TARDIS. Once she falls asleep there and he cups her gently in his hands and takes her to her room.
But them going on adventures and getting stuck where Rose would stick out, so she shifts and hides in his pocket, or the collar of his coat.