#depending on how we go

I would love to see the Doctor’s regression be a multistage process - Thirteen regenerating into TenThree, and then another past Doctor, and another one, maybe even a Master, playing with the lack of regeneration limit. It’d be great fun, a good story, and excellent audience-bait.

But definitely I think it needs to start with TenThree, and also have them as the form of Doctor with the most time on-screen, so as to metaphorically complete/change both “Ten” and “Thirteen’s” stories from their Bad Endings to New Game+ Good Endings.

Because the Doctor regenerated into Ten out of the fear that Rose (who had become essential to him) would abandon him - he needed to be pretty, needed to be shiny, needed to be outwardly nice and personable etc. etc. regardless of the fact this was clearly untrue. And coming after Thirteen would mean TenThree would also almost certainly be born out of the same desperate desire to cling to the person they loved, who’d become essential to them (which they then lose anyway causing a complete crash).

So how perfect to have another Rose, who herself knows what it’s like to step into that sort of big change, be the one to help the Doctor (TenThree specifically) through this baggage, and help them regenerate into someone who reflects themselves for themselves, not for other people or guided by fear.