Squad is fine except for 2 positions.
1) Rightback. Essential.
2) Playmaker/DM/CM in midfield, depending on where do we see Messi in the future. If we move him back to the right once (or if) we sign a great RB we need a playmaker in midfield, or an extremely well rounded player who could be very good in several areas of the game, including on the ball and passing qualities. Or, if we see Messi as an attacking midfielder a la Totti from now on, we need a more defensive player imo. A player who could form a partnership with Busquets and could allow Messi a midfield position even with his almost non existend defensive participation. Messi would be the 10, so, the player we should sign needs to be both a 6 and an 8 (but more on the defensive side), allowing us to switch from a 2-1 (Busi + new player - Messi on top) midfield, to a 1-2 midfield when we need to.
So, right back, and a certain type of midfielder depending on Messi's future role.
And a new manager, one who plays more fluid football, and is not as restrictive regarding players' movements in positional play. Basically, when one player has to ball, I want to see his teammates doing something to allow themselves to be in a position to receive a pass. That means moving around and not being confined in a restricted space to operate in. The more restricted the space you're allowed to move in is, the easier it is for the opponent to stop you from receiving the ball in dangerous positions. If you are not allowed to leave your zone and look for the ball, it's hell of a lot easier for opponents to mark you, or close down passing lanes to you. They just have to position themselves cleverly and it becomes increasingly hard for us have a good forward pogression when in possession. All of this because Lucho has a very rigid understanding of positional play.
And it's not that you can't actually play a more rigid system. You can, if you have the right players for it. But we don't. And not only that we don't, but we are even experimenting with solutions like Roberto at RB, while playing a type of rigid football in which one of the inevitable outcomes is the isolation of players in their respective positions both on and off the ball.
Sure, players can play in a system in which they have to cover their zones and do their jobs in a more individual way (due to the incresed spaces between players, everything you do wrong, has the potential to be deadly for your team, simply because there is a constant lack of support for players who activate in more crowded areas), but at least make sure the players and the positions you play them in are a perfect match. Basically, you need to make sure your players have the necessary qualities to perform their designated role without the support of a more fluid system, where covering zones and defending spaces becomes a collective tactic, which hides away some flaws in your players, that otherwise would have been more visible and more exposed. The way we are playing exposes flaws in Busi's game for example that he always had imo, but the system made sure they didn't manifest.
Problem is we need starting material players. That could well mean something close to 100m for only 2 players. If not more. But we could actually play a lot better with the same squad unde a more creative manager.