Here’s my new headcanon. Donna has timelord DNA. I think when she dies, she’ll regenerate instead. She’s never really forgotten. The memories are still there. Some stuff could be leaking through. She’s still the DoctorDonna. So there’s plenty of reason that she could adapt to the Timelord DNA inside her. Slowly, over time. Now I know she only has one heart, so that could put a damper on the regen but if she can adapt to the timelord knowledge, I’m sure she could find a way around that.
How many hearts does River have?