I am 99% sure that Messi will leave. He would probably go to Manchester City, little chance of going to PSG, but at Barcelona he shouldn't stay.
Well, the current Barcelona is weak, with the departure of Messi, I believe that a World Class player or two players should come to replace him
Barcelona will still need to invest in another 2 or 3 players more to start. The president made many bad decisions,dismiss Suarez was ridiculous, he's being the best player in the Spanish league and has everything to take Atletico to the championship, Barcelona lost a lot without him. It would also be necessary to sign a center forward among the best in the world for his place.