NOT A NEW THEORY AND NOT THE MOST POPULAR.
Moffat missed the biggest opportunity to bring an arc full circle and the thought of it makes me want to throw up.
Let me start by saying I think this theory still holds true even with the way the story has progressed.
DONNA NOBLE IS THE HYBRID.
Hands down zero doubts.
That's why 12 chose his face.
That's why he lied and lied and went through the whole confession dial thing.
1 he was mad about Clara sure.
2 He was protecting Donna (obviously)
That's why fate had their timelines all twisted together not only for the whole davros destroys all of creation thing but because he was always intertwined with the hybrid.
That's the real reason he erased her memories and him asking you know what this means and her saying yeah hurts so much more because she knew why her memories had to be erased because the prophecy had been passed down in Timelord history only to sprout it's beginnings there.
This brings me to MOFFATS MISSED OPPORTUNITIES:
Donna Noble or rather her new hybrid form had her memories erased to protect her.
Wilf said she seems okay but every once in a while he sees this look on her face like she is so sad... But she can't remember why.
Memories erased and Timelord part of her suppressed. Things were still leaking through. The doctor knew his psychic memory block wasn't up to par. This was furthered by his need to use the device on clara for the full memory purge.
So my theory goes; Little by little donna starts becoming clever, and sad. She can't understand what is missing what is wrong with her. Eventually being around her mundane life doesn't cut it for her anymore and she begins searching for answers elsewhere.
Always trying to fill that hole the doctor left inside her.
"The prophecy identified the Hybrid as a being born of two warrior races, and said that one day the Hybrid would stand in the ruins of Gallifrey and "unravel the Web of Time and destroy a billion billion hearts to heal its own."
She couldn't remember she couldn't remember, and every time she had a breakthrough she would black out.
The only single thing that kept bleeding through was "Timelord" "Timelord" "Timelord" "Timelord"
But what did it mean? What was a Timelord?
All she knew was that she had to keep looking.
This nagging question lead her into many dangerous instances with many other alien races. All of which she was some how able to out witt with information she had no idea how she knew.
Eventually she finds herself on Gallifrey standing before the cloysters still looking for answers.
How many lives were lost? how far gone was the once donna noble at this point? Just to get her here. The Timelord home planet. In the cloysters where the mass of their great knowledge inevitably ends up.
She stands there in tears. Almost unrecognizable as the Donna Noble we love.
The 12th Doctor's voice can be heard from behind her:
"Donna Noble I am so so sorry."