I honestly think that Busquets is capable of doing Xavi's job in the midfield. He has the qualities. It's only because he is being restricted in his role at DM that we haven't seen his other qualities. Though he is less agile than Xavi, he is still technically gifted, has the vision and has the ability to play the ball out of pressure. I hope Luis will test him in Xavi's role at some point next season.
With that being said, I still hope he stays, not as a starter though (if Cesc stays). A Giggs role will suit him perfectly.