I'm just a trash for Piper Mclean and Jason Grace, you no have idea.
Look all the "let me fix you, Dean" scenes when castiel cures Dean. There's a white-blue light-grace-power. Now look at the last one, is kinda orange, like the smoke-grace-baby-energy-power-thing. When cas dies, it's the same white-blue light, as expected, but if, somehow, he's had two "energys", after the scene when Dagon died, he can still be alive, without the angel part, but probably strong then ever.