The only possible thing is that adding the depth to the winger position. Leao is a both-footed player. Having him can add the dept in Barcelona's winger position(RW/LW), but sadly Allegri has responded it by blocking his transfer. So he still remains at Milan, and won't leave in summer.
Honestly, I am glad that Barcelona's deal with Nico Williams is not final. Barcelona now needs a worthy replacement for Lewandowski. Because it is certain that Lewa will leave Barcelona in the next few seasons. Lewa is almost 37 years old now. Nico is a talented player, no doubt about it. But he is not able to play in Lewa's position. At the same time, Nico does not have the ability to score as many goals as Lewandowski.
You are right that Leao can be a good choice for Barcelona. He can play in Lewandowski's position. Rafael Leao also has the ability to attack quickly. So Leao's performance with Barcelona will be even better.