I wonder why Jody Whittaker is stepping down? For the first female "Dr.", I think she's doing an awesome job. I'd love to see her stay another season.
David Tennant coming back? That also would be awesome!!! I doubt it though, he's got too many other "irons in the fire" to be able to devote the right amount of time to play the Dr. again.
If they get a "big name" to play the Dr., it will go shit turds up. Thats what happened with the Batman franchise.......DC thought it was "cute" to give the part to every dickweek that comes along and wants to wear the Batsuit. As a result, they fucked it all up.........heinously. They did the same thing to Wonder Woman. And the same to Superman.
Keep the part small with small actors. Don't get any big heads in their going full fucking Diva and screwing the character up for all eternity. It was bad enough they let some fuktard put Capaldi in the role!!