I thought STscoop had mentioned Toby Whithouse at one point was a front runner? Or at least a rumoured front runner? My apologies if I'm misremembering.
He used to mention Whithouse, now he mentions Chibnall, next week he might mention someone else...
Both Whithouse and Chibnell are obvious choices, as is Gatiss (although I think he has less producer experience.) You could throw in the Merlin creators and probably a few others...and this assumes it'll be an obvious choice.
Me I'd quite like to see Joe Aherne in charge given I've always liked the stuff he's done (even if Crickley Hall was a bit lacking).
Or here's a random idea...Noel Clarke...