The central midfield is in danger of being very crowded next season, if both a midfielder is signed and Aleñá is promoted, and only Turan is sold. Roberto could be back in midfield, too. If only Gomes was never signed. It could become another mess of a position (excluding Iniesta) next season.
Only if a coach with a CLEAR philosophy regarding the midfield, with three players much tighter and much more internally involved than what we have seen post-Xavi (only see Eusebio in that regard of those available, tbh), then I see a signing like Verratti or Eriksen (both lying deep, choice between more control and more attacking danger) being of the utmost importance. Would prefer Isco over both if that wasn't even more impossible than Verratti (for now, before his contact runs out in 2018).
With Unzue or Valverde, it's less clear. Maybe physique is more important for those two, and then maybe Fabinho (or just no signing at all, just sell Arda and promote Aleñá) would fit them better. Only time will tell at this stage. One thing is for sure, it won't be cheap, and we also need a right back...