Song is a quality player, adding him would improve the team, Arsenal would be insane to let him go. The way he could provide coverage for both DM and CB would probably be to free up Busquets to play CB when needed. He could be at his best in the 3-4-3.
He offers more assists and scoring than Busquets does, and he can do amazing things dribbling. What I don't know is if a player of this quality could settle for the amount of playing time he will get behind the world's best DM. Busquets does not get hurt and is able to play big minutes.
The Barca regulars outside the first 11 next year could include Pique, Adriano, Song, Thiago, Cesc, and Pedro. That's the type of depth that can see you to trophies in all three competitions.