Barzagli is one step away from retirement. I think we can't rely on him anymore.
Rugani, Chiellini, and Bonucci. That means we're down to 3 CBs. Benatia is probably leaving after this season, so even if we bring in Romero (who's probably going to spend next season on loan @ Genoa), we're going to be incredibly short in that department. Chiellini isn't getting any younger and is injury prone (he's still a beast tho) and Bonucci is as unreliable as they come. Let's say we get De Ligt. That still leaves us with 4 CBs, which might still not be enough. I'm afraid Sandro might have to be sacrificed if the Marcelo rumors are true.
In midfield, I'd seriously consider selling Pjanic if we want to upgrade beyond Ramsey (provided we can't make good money on those sales I mentioned earlier). Khedira should be gone come season's end too.
In attack, we should maintain status quo, and then go for a super talent in 2020.