Sell Messi to PSG, City, or whoever else can afford him. Sell Griezmann to anybody who will take him, even at a huge loss, as his wages are terrible and he isn't the player to build around. Suarez, Vidal, Rakitic are out of contract in 2021 so take anything you can get or let them walk next summer. Tell Busquets, Alba, and Pique that they can find new clubs or hang around as veterans to guide the younger players, but they aren't going to be centerpieces or regular starters going forward. Fire Semedo into the sun.
With whatever funds you raise, buy two new fullbacks and a striker. Hire a progressive manager like Nagelsmann or Pochettino who will actually implement a real tactical system and have him build the team around the new additions plus Dembele, Coutinho, Pjanic, De Jong, Lenglet, Todibo, Trincao, and Ter Stegen.