I understand completely why Canna left despite the fact that I was very unhappy about it. I would be thrilled if he came back and would welcome him. He would add a lot to the team immediately and help our younger future defenders. Canna is more than just a great defender, he has outstanding leadership qualities. With him on the field I would breathe a lot easier with Juve back in Serie A. And no I don't think he's too old. He's in great condition and has another good two years before he slows down. What he loses in speed he makes up for with experience in reading the plays/players.
Don't forget he struggled early with Real because he had to adjust to a new team, a new city, and a new country. This is not always a seamless transition.
Canna has only always said good things about Juve and the Juventus fans. He is neither a mercenary not a traitor. Say what you want, he did give a lot to Juventus while he was with us.