I suppose if Barcelona were ever to sell Messi the near future would probably be the time to do it, they have a strong attacking presence and I really doubt Messi will get better with age at this point. Seems like he's a major factor in the clubs decision making though, seemed last season like Martino was hired based on this relationship. If they were to sell him, they'd be set for a very long time transferwise.
That being said, as if this is ever going to happen with FFP!