Just wanted to mention three names; Aymeric Laporte (buy-out clause €50m), José Gayá and Thiago Alcantara. They would cost Barcelona around €120m in total and could replace Adriano, Mathieu, Douglas and Song. José Gayá isn't a priority signing because Alejandro Grimaldo should/could be promoted to the first team to get used to the higher tempo. However, Laporte is an amazing young defender (one of the best U23-defenders in the world) and everyone knows Thiago. He can control the game like Xavi used to do for us. Besides that, he would probably be cheaper than players like Koke, Verratti or Pogba. Our team would look like this (after the deserved promotion of Samper and Grimaldo):
GK: Bravo, Ter Stegen, Masip
RB: Dani Alves, Sergi Roberto, Aleix Vidal
CB: Piqué, Mascherano, Laporte, Vermaelen, Bartra
LB: Jordi Alba, Grimaldo (Vermaelen can play back-up until Grimaldo is ready)
RCM: Rakitic, Thiago (Sergi Roberto as back-up)
CM: Busquets, Samper (Mascherano as back-up)
LCM: Iniesta, Rafinha, (Denis Suárez will return after this season)
RW: Messi, Sandro, (try to keep Pedro as main back-up for our front three)
ST: Suárez
LW: Neymar, Munir (Rafinha as back-up)
Edit: Arda Turan can play as back-up for Iniesta, Messi and Neymar. Totally forgot about him!