Posting this again with some new information by some other anon:
211 teased us with a preview of a third doujin before disappearing (pic related). I wonder what became of it and why he never posted the full thing
He also posted this novel, that's supposed to go with the doujins:
Mikasa wake up in the dark,pain runing through the ribs that were not fully recovered.
Trying to not move so much because of the pain she looked at her surroundings.
It was in a small room,she could hear two persons sleeping next to her.
Moving her neck a little she saw Sasha and Historia sleeping.
Mikasa finally understood that she was dreaming and that is in the real world now.
She picked up her scarf that was next to the bed and wrapped it in her neck as usual.
Then she coffered her face with the scarf raising it with her hands.
However there was a anxiety in her chest that was not going away.
Slowly,she get out of the bed.
With the horrible vision still in her mind and wanting to be sure it was just a dream,Mikasa left the room.
Eren,that was sitting in his bed,jumped out of it at the sound of the door getting violently opened.
He looks up at the door in time of seeing a familiar figure emerging from the corridor.>Mikasa?You really scared me.
Giving no chance to Eren react,Mikasa punch
him in the chest.
He keeps coughing for some time.>Do you had a scary dream?
It was a low,gentle voice.
Becoming to cry,Mikasa nodded quietly.>I see...
(rest of it isn't translated, and has been deleted from pixiv, source: http://pixiv.net/novel/show.php?id=3221284&mode=text#1
Some pretty interesting stuff. 211 will always be a mystery.