Summary: Cas is too late.
Cas being sent back to heaven by Dean could only mean one thing: He intended to say yes to Michael. Cas just had to get there before Dean did something he would regret.
"Our father, who art in Heaven, hallowed be thy name-" There. That had to be Dean reaching out to Zachariah. Cas transported himself from heaven to the place of the prayer.
The forced trip to heaven, however affected Castiel's powers and he ended up taking himself a block away from where Dean was. He only saw Zachariah and Dean a block away for a second before they were whisked away to God knows where.
"Dean!" Cas shouted, but he was already gone.
AN: Let me know how far you want me to take this, because I can take this all the way to the apocalypse or just end it here. Review and let me know!