I agree that it would be interesting but I would hate for him to become human after getting his angelic powers back and becoming もっと見る powerfull. It would be a cruel twist of fate. And I don't think I want to see him become human. He's perfect as an エンジェル and should remain so.
I would highly doubt it. He just got a promotion of sorts for 天使 so it seems unlikely he would fall now...but I don't know, that's up to the writers. Personally, I like Cas the エンジェル so I sincerely hope he never falls.