Well, i'm sure it would take exactly what Williams is to bring him back. Young, inexpensive, talented, young players. Simon Gagne would probably be of interest to Carolina. Jeff O'Neill could be of interest to Philidelphia as well.
Back in reality though, i'd say the chances are slim to none and slim just left town. Williams did great in his stint with Carolina and is an integral part of Carolina's future. Teams don't often trade players only to turn around and trade back for them at the end of the season.
The fact that Philly fans miss him so much speaks volumes as to the type of player he is though, and we're glad to have him. He's certainly not lacking in the fan department here in raleigh either. We're taking care of him.