Based on the way things seem to happen in the show, I would guess that somewhere next season, Castiel will recover his powers. How many times has he gotten lucky so far? I faintly remember Lucifer (as Sam) snapping his fingers and blowing Castiel to bits, but he returned, still wearing the same vessel. But, he will linger and be hilarious trying to figure out how to adjust for the first 75% of the season. Perhaps he will discover more things that humans enjoy, like had happened with alcohol, pain killers, porn, and burgers. (Burgers make him VERY happy.)
The next thought is more of an I HOPE! I thought Jim Beaver did a good job portraying Bobby. I'd like to see him return to the show. They've given him a few mini revivals. First he was killed, then he attached himself to his flask and helped the brothers out as a ghost, then Sam rescued his soul from Hell, so how will he be revived next? My hopes are that he was turned into an angel after his soul made it to heaven. Then, Metatron banished all the angels from heaven, therefore returning Bobby to the show. So, my hopes would be, that Castiel continues to tag along with Sam and Dean, while making a fool of himself, and their new guardian angel will be Bobby. Then he'll get to teach Garth how to really act like him.
Last thought, and I saw someone else mention this as well. I think when the angels were cast out of hell, it would have been the same situation as with Lucifer. They will still have powers, but they won't be able to return to their home. It's possible that they will linger around like Castiel, as another person mentioned, but since they were at war in Heaven, I would assume that they will continue that war on Earth. Some of them will grow vengeful for what happened, and others may continue to fight for what's right.
Meanwhile, Dean, Sam, Castiel the Lingerer, Bobby the Angel, and Kevin will try to avoid getting themselves killed, and tried to figure out how to banish all demons to hell and all angels to heaven, at the same time, without having to sacrifice themselves. If the next season is the series finale, I hope a God character makes an appearance and sets things the way the he likes it again.