Why I ponder "Doctor Who" at night, I do not know. If it's worth anything, I also ponder public transportation networks.
Anyhoo, I figured out how to bring Donna back should the desire to do so ever exist. It was hinted at in the episode itself. Donna had a large ring on her finger when the half-Doctor was created. In the last glimpse of Donna that we see, that ring gleams. Also, when the Doctor returns ignorant Donna to her parents he inroduces himself as John Smith, the name he had used when he was human and hiding his time lord configuration in a watch in "Human Nature."
So, in your bringing-back-Donna episode, you simply have Donna experience a mild electric shock (that's what kick-started the change when fighting the Daleks) and suddenly the dormant Doctor in her mind comes to the fore. In the time that he's been floating in her sub-conscious, he's been able to work out a plan -- all inspired by when the Doctor introduced himself as John Smith. He simply stores his conscience in Donna's ring, which he is able to do because the Doctor's regenerative energy passed through it when the DoctorDonna was created. Easy peasy lemon squeezy.