The first priority is obviously a left-winger. We urgently need to give Rapha more pauses, or Lamine, so Rapha can take over the right side without the team losing speed. Ferran is too slow for that role, Pau and Fati should leave the club in the summer, as they're simply not good enough.
If Flick sees Ferran as Lewy's backup and rotation, I can live with that. Not my ideal solution, but we can't be picky right now and though Ferran has clear weaknesses, he's sufficient for now. So far we didn't lose games because of him.
There are reports that Flick sees Dani Rodriguez in Pau Victor's role next season. If he trusts him, than one additional LW is enough to strengthen the attack.
Rapha/new LW Lewy/Ferran Lamine/Dani Rodriguez
We don't need to do anything in the midfield obviously. We're more than filled here with quality and quantity. Olmo, Fermin, Gavi, Pedri, Casado, Bernal and if necessary Garcia, Christensen, even if Christensen will leave the club. That makes 8 (7) players for 3 positions. More than enough. Torre should leave the club.
Olmo/Fermin/Gavi
Pedri/Casado Frenkie/Bernal (Garcia, Christensen)
The defense will depend on Flick and how much he trusts the current squad. Either Flick trusts Fort and gives him more minutes, or the club needs to sell him and bring a new player to give Kounde rotations. Shouldn't be a competition type though, as Kounde is clearly ahead of most RBs in the world and doesn't need many pauses being in his prime and being a physical beast with the most minutes in Europe and his first injury since 2,5 years.
LB, Flick definitely trusts Martin, so in my opinion there isn't any need to do something. Balde keeps improving, Martin showed he can play the most important games. Yes, he makes mistakes, but he's from La-Masia, earns minimum wage, won't ever be toxic for not having too many minutes and his ceiling is not reached yet, he might surprise us next season after having earned much experience in the latest games.
CB depends also on Flick and on how much he trusts Araujo. We surely can't hope for Martinez to perform another season like this, he isn't getting any younger and is usually injury prone. Cubarsi will need his pauses too, as he was clearly overplayed in the end of the season. If Christensen leaves the club, then we need a new CB even if Araujo stays.
Garcia and Christensen can help here as well, but a new CB must suit Flick's system and who you can start in big games without stomach ache.
Balde/Martin Martinez/Araujo or new CB Cubarsi/Christensen/Garcia Kounde/Fort or new RB
In the goal, it'll depend if Szczeny will extend for another year or not. MAtS is first captain and will definitely be a starter next season, where most people here like it or not. If only just for the reason to see how he'll do with Flick's system. If Szczeny will leave the club, Deco should definitely get a keeper with high potential to take over from MAtS in around 2 years. Pena should leave the club regardless.
MAtS / Szczesny or a new keeper
That makes 24-25 players.
In summary go big on LW, rest depends on Flick, Christensen, Araujo and Szczesny. Keep saving big money for 2026 summer, where we'll need a 100m+ striker.