Not many players improve us if I'm honest. You could look at the winger situation, but there's very few world class wingers at the moment. The top ones seem to be at PSG. Rather keep Doku, Bobb and Savinho and develop them than spending a fortune on Real Madrid's benchwarmers. Although, if Bradley Barcola became available I'd bring him in and add to what we already have.
Possibly one player who would improve us is Bastoni. Centre back area still seems a weaker position for us than in recent years. Current Dias and Gvardiol doesn't seem as solid as Prime Kompany/Stones or Dias of a few seasons ago/Prime Stones.
Players like Eze, Gibbs White and Gordon are very good players, but they seem like very good squad players rather than players who immediately improve the first 11.