Ideally we would want (in order of importance):
A quality leader at CB (Bonucci, Pique, Alderweireld, Laporte etc)
Between 2 and 4 highly functional fullbacks (Rose, Grimaldo, Bernat, Kimmich, Henrichs, Fabinho, Mendy etc)
A deep lying playmaker (Weigl, Paredes, Kroos, Busquets, Dahoud)
A good sweeper keeper (Ederson)
Another quick, talented forward who can play across the front line (Coman, Mbappé, Dybala, Sanchez)
A tough defensive midfielder (Carvalho, Kessie)
A "plan b" striker (Llorente, Gomez)
A younger silva-type (Bernardo Silva, Isco)
However that's way too much of an overhaul. I don't see any top class regista types being available, but we have Gündogan, Fernandinho and Garcia coming through. Isco seems like a top target so I see that being a deal of opportunity to get such a top quality young player at a reasonable price. Another forward would be necessary if Agüero leaves, otherwise we could definitely make do with Kun/Gabriel/Sané/Sterling/Nolito + one (or 2) of our talented youngsters (Ünal, Moreno, Roberts, Sancho). We NEED to inject a bit more quality in to our defence, personally would like to see Sagna stay another year and Kolarov go it looks like it may be the other way around. We need a top top quality centre-back, Bonucci would be perfect although how likely he is to leave Juve is uncertain.
Bonucci
Kimmich/Fabinho
Rose
Henrichs
Ederson
Isco
That would be an excellent window for me.