First, we can't sell Messi, we gave him the keys to leave whenever he wants just with a warning a year in advance.
Alba, Busquets, Rakitic, Suarez, Sergi Roberto, Umtiti and Vidal have to leave if you want to rebuild, we can't compromise to give minutes to players that are obviously way past it and want to play based on their legacy alone, keeping them is the root of any future problem.
Pique can stay but as the 3rd/4th CB option, he's still valid and at least he says things clearly, him, Ter Stegen and Messi are the only players capable of passing the torch between the old guard and the new players to come.
Semedo, Firpo and Dembele deserve a second chance, they haven't been up to the level we thought they'd reach, but honestly, no one escapes the general poor level of this roster, they need players that can put more of a fight against them in their spot.
Ter Stegen, Lenglet, De Jong and Griezmann should be the pillars for the next ~4 seasons, these players should be above any reasonable doubt of them being Barcelona worthy, even if Griezmann has struggled this year, he's been used as a scapegoat since day 1.
Puig, Fati, Aleña, Trincao or Pedri should start seeing decent minutes on a consistent basis, but never be weighted down by the responsabilities that their seniors should have.
Messi has to either have enough good players that cover for him in defense to keep playing his amount of minutes, or start playing less but performing at 100% in the minutes he can.
Basically, we need elections this same year (Bartomeu shouldn't be able to milk another year of this), a new DoF (Abidal is done), a new staff with a manager with total control over the squad and without the pressure of winning everything now, 2 FB's (one on each side), 2 or 3 CB's, a reserve DM, a CM, a winger and a striker. They don't need to be world class, they just need to fit the game plan of the new manager, Messi, Griezmann and De Jong are world class enough to sustain the team in the short term.