I think, the management should not touch the defensive line this summer. I'm not a big Bonucci fan, but I'd be against his sale in the next transfer window, whether it's to replace him or to rely on such a sale to finance some important deal for other line. A proper replacement would mean a lot of additional money spent at the expense of the midfield, which is unacceptable.
In order to finally succeed, this glorious management team has to have one simple task to focus on: to improve the midfield. Do not touch anything else. Do not even think about anything else. Just the midfield. It means, sell Rabiot, Ramsey, Bernardeschi, Douglas Costa, and replace at least two of them with top level midfielders.