Will say it again, Messi is immense drifting just behind a striker. Leave him alone, let him go where he feels he needs to go, and he becomes even more dangerous. If we can get the others like Neymar and Iniesta to find consistent form again, then the pressure is taken off Messi, and he can do what he does best. This season, Messi has been under too much pressure to carry the team, and his performances were up and down. Not hos fault, and when the fans gets on his back, then its a tougher ask.
Central defence is priority, but an experienced attacking duo of Reus and Aguero wont be bad. Xavi days are coming to and end as a player, and Neymar can slot in there. For me following i would sign:
Ter Tegen
Reus
Luiz
Aguero
Cuadrado
Javi Martinez
Leighton Baines
Ter Stegan
Martinez/Cuadrado Luiz Pique Alba/Baines
Busquets/Martinez
Neymar/Cuadrado Reus/Cesc Iniesta/Pedro
Messi
Aguero/Reus/Neymar
For me that is a very flexible team, with Martinez and Luiz able to fill in at Center Back, if Pique is injured or suspended. Reus can play any where in the front line, so can Messi and Neymar.
On the bench there will be Cesc, Rafinha, Pedro, GK, Cuadrado, and whoever doesnt start.
2 central defenders, a right back, and 2 versatile attackers plus Ter Stegan and i think we are good to go.