I would go for either of Quintero/Felipe Anderso (both different type of players, one is a Left footed Isco-ish player, and the other is a Kaká-is player in terms of explosiveness). Quintero's situation in Porto is not the ideal one, he's struggling to adapt and he could come here on loan with an option. Felipe Anderson would be more tricky because Lotito hate us.
And one of Dybala/Vietto (I could be really wishful thinking here with Dybala, but if reports are correct, we really are pushing for him, and if Suarez autobiography is legit, when he said we bidded for him and he stayed because of Gerard's advice, we are not afraid of spending for the right player.) Vietto's release clause is around 20 m according to recent reports (i believe a statement made by his agent).