First of all, I have to say that I won't include players like Paulino Alcantara or Ricardo Zamora, because I have seen some rare (which is not enough) footage of them. I will just go for players who were good at Barcelona and not somewhere else.
In goal I'd choose between Valdes, Zubi and Ramallets. I would have liked to take Ramallets because he would be one from El Barça de les Cinc Copes but I think Valdes ist just more suited to this style of playing.
At right back I will take Dani Alves, in my opinion he is one of the best players in this position - someone who can play three positions in one because he is such a force of nature. On the left side I have the pick between Joan Segarra and Sergi Barjuan. Sergi was probably the slightly worse player and Segarra would walk into the side if he was a natural left-foot, yet I am not too sure what to think about it tactically. Even Abidal could make a challenge though, in my opinion he was(is!) a frightenting player and is the third person to consider on left back. Sigfrid Gracia and Gio are players who also have a place in my heart, though I will go for Segarra. He was also really good on the ball, it surely won't be a problem.
In the middle I will take Koeman and ...
Is it Puyol or shall I take Pique? No, but he will be one day in this team for sure. Puyol for me. Alexanko, Migueli and Ferran have to be benched too.
Central I have Busquets, Guardiola and Iniesta, though I would probably regret to bench Xavi, Laudrup, Maradona, Schuster, Neeskens, Deco, ...
Laudrup or Maradona for instance could take over Iniesta's spot (but he is my alltime favorite @Barça), Deco, Schuster and Xavi could take Guardiola's place and ... no one could take Busquets' place, maybe Guardiola or even Koeman and Yaya(?). But not with his style.
In attack there are also many many players: Evaristo, Cesar, Kocsis, Eto'o, Rivaldo, Krankl, Quini, Stoichkov, Ronaldo, Romario ... but they can't play all, can they?
I will go for Messi on the right side, Kubala in the middle and on the left side Cruijff.
I also had the idea to put Cruijff into the central spot and Ronaldinho on the left wing. So Kubala can occupy the place when Cruijff goes back, they can pull the strings together and on the wings there are two of the most gifted players in history. Though Ronaldinho needs to run and press if he wanted to keep his place - but he didn't! So I went for Messi - Kubala - Cruijff. Even if it's a shame to see two of the best players ever a little bit wasted.