Hmm, would that be space-masturbation or space-incest? I want to see him take Alt!Universe!Donna and bitter twenty-seven year old divorcé Rose, find some sort of TARDIS-like device, and go kinda evil and duke it out with our Doctor for the use of the TARDIS... until he's old, at least. Then he might give it back.

Then they try to figure out a way to save the Other Donna.

And Rose wants half of the Tardis legally due to her, as that judge on the telly informed them in their televised divorce proceedings.

They also have other adventures.