I would first slowly replace the older players, such as Puyol with younger, but still very solid, younger players. Thiago Silva, Chiellini, Hummels would all be great first team picks, whereas Papadoupolous (sp.) and Munisia (In Barca reserve. Will turn into fantastic ball playing defender) will make a great defensive partnership in the future.
Left back is probably Barca's weakest area, and so it would be a good idea to replace Maxwell. Lahm, Crisito (sp.) and Jordi Alba are great left backs, but it would probably be a better idea to wait a couple of years until a wonderkid left back appears. You could retain Chiellini to become natural at left back - he'd be great there! You may want to sign a wonderkid right back, such as Romario, to eventually replace Dani Alves.
Then goalkeepers. Pinto is hardly worthy or being the back up keeper, so I would sign ter-Stegan. He's a quality wonderkid, who starts of pretty solid! He'll only keep improving, but will cost 20mil. No doubt, he will be worth it in a couple of years time - once he's kicked out Valdes from the starting XI!
So, the back four could shape up like this: Valdes/ter-Stegan (GK), Chiellini (LB), Thiago Silva(CB - Stopper), Pique(CB - Ball Playing Defender), Dani Alves(RB.)
The rest of the team is pretty youthful, but you still may want to be on the look out for the latest talents so your rivals don't snap them up! Mascherano will soon need replacing, so you'll need to find a suitable replacement. M'Vila would excel at that role, and he's pretty solid - even to begin with! He's also a very reliable player. Xavi - well you have Fabregas and Thiago Alcantara, as well as Rafinha if he develops well. You could also sign Felipe Anderson, Jack Rodwell, or Kyle Barkley - although the latter two are different players, they'd no doubt provide excellent cover. You could also sign Gotze, or/and Hazard to provide good competition for Iniesta or whoever your wingers are. Finally, David Villa is the only oldish player left. There are loads of great players to replace, and probably even surpass, him. Aguero, Cavani are first team replacements, while you may also want to consider Iker Muniain (He's Spanish too), and Mbaye Niang.
Barca are a great team, so really, all you need to do is shape up the defense, and replace some of the older players. Other than that, just be on the lookout for the next Lionel Messi's, so you can continue domination world football.
