Rex Ryan is a possibility, but I would think that San Diego would prefer keeping their offensive system as intact as possible, which means bringing in Norv Turner, who uses the Air Coryell system.
As for Carroll, I don't believe for a minute that he wouldn't return to the NFL. The reason for his mediocre NFL record is that he coached for teams with horrible GMs who simply did not give him any sort of talent. Look up New England's 1998 draft roster and you'll see what I mean. If Carroll is offered full control of an NFL team, you can bet he'd leave USC. He almost left USC for the Dolphins a month ago--but didn't, because we intended to give more authority to Randy Mueller.
I follow USC football closely and Pete is not interested. He is going to make college football history coaching the Trojans. We have not seen the best from them yet. The Trojans are going to be frightening next year.