As a result of this discussion, myself and crossroadscastiel got onto the subject of Dean being powered up by angel Grace and Cas ultimately transferring his stolen Grace to Dean as a means of saving him from potentially becoming a demon/Knight of Hell as a result of the effects of the Mark of Cain.
I honestly don’t think it’s that far fetched of an idea. The number of hints there have been throughout the series that Dean is more than a simple human are hard to ignore, especially when you lay it all out in front of you. It’s perfectly plausible that the antidote to the grip of darkness on Dean’s soul is to force it out by Grace.
The reason this is so plausible is because of what happened to Castiel when he lost his Grace, then stole the Grace of another. When Cas died all three times, his body was remade in the form of Jimmy Novak. When Cas lost his Grace, they never explained how he was able to remain in Jimmy Novak’s visage. When any other angel, or demon for that matter, has their Grace, their essence, taken from them, their vessel either dies or the host takes back control. This didn’t happen with Cas.
Castiel himself is the essence of his Grace, yet that Grace was taken. Therefore, we must assume that when God remade Cas, he made this body just for him with room for a soul to grow so that he could exist as a human when the time came. Cas needs Grace to have angelic power, and he stole the Grace of another angel without bringing the essence of that angel with him. It wasn’t possession, it was transference. Thus, the Grace inside of Cas is sharing space with his own soul, binding to it, feeding off of it. If you look at this way, it is doing the exact same thing to Cas that the Mark is doing to Dean. Both are foreign substances that are poison to their souls. Both toxins are burning them out.
Now, by this vein, say Castiel transfers his stolen Grace to Dean in order save him. The reason Cas was able to remain himself with another angel’s Grace inside of him was because the essence that is Castiel is divine. Transcendental, if you will - created for this purpose. So, Grace is transferred to Dean making him into an angel, but how does Dean remain Dean and not take on the essence of the angel whom the Grace belongs to?
Sure, Dean is Michael’s true vessel, but does he become Michael by receiving any angel’s Grace? No. Why? Because Dean’s soul, as it is, is already transcendental.
How do you think Sam was able to take back control from Lucifer? That doesn’t just happen - power of love or not. The soul has to be created for the purpose of binding to Grace in a way that the Grace becomes subordinate to the soul, to the essence of the human carrying it.
So if Dean takes on another angel’s Grace, he’s still Dean. A little more immortal, but still Dean. And it would work much better in the story of two brothers, saving people and hunting things. Demon!Dean doesn’t exactly strike me as the “saving people” type. As an angel, Dean can still do the work he was put upon the Earth to do, and do it better than he did before. All these things that have happened over the years - Dean’s destiny - could have been leading here. It would fit perfectly as number 7 on the list of universal alterations by the Will of God that I discussed here.
Now, as for Cas directly transferring the Grace he currently has to Dean - I don’t think that would work. Cas’ stolen Grace is just another form of poison. If added to the toxins already tearing Dean’s soul apart, it would most likely only do more damage.
But if Castiel found his true Grace and gave it to Dean - that would be something.
Cas would choose to give up his Grace and become human again because Dean’s life is worth it to him. He would give up his Grace in a second if that was the antidote to Dean’s poison. Then Dean would become an angel in a monumental role reversal. It wouldn’t be his choice, but maybe something in the divinity of Grace will awaken the transcendental power he’s had inside all along. Or perhaps it will be a means of purifying Dean’s soul, absolving him from his sins and his guilt, and allowing him to start fresh. He would be an angel, but the Grace would bind to his soul, leaving intact everything that is Dean Winchester, love for humanity and free will and all.
I think that would make a hell of a story.