i see your point. a world class CB is definitely up there on the list although i don't know who barca could buy at a reasonable price...
Centre backs that are out there that I believe are Barca quality and that are attainable are:-
Hummels, Subotic, Sakho, Kompany could be targetted. I mean I'd seriously consider offering in part exchange some players like Thiago, Afellay and Adriano in part exchange for Kompany. Or just spend £20m on Hummels and have a world class centre back for the next 10 years and if Pique can get his mojo back then you'd have a brilliant central defensive partnership. Or even, you know, exchange Pique for Kompany and you'd the definitely have the best central defence in the world with Hummels, Kompany and Puyol.
Plus whilst Mascherano is a totally awesome player at centre back, he is a
BEAST, in his proper and correct position of defensive midfield and rotate between JM and Busquets in that role and thus keeping both of them more fresh throughout the season. Which means the likes of Xavi can have a rest now and again and you can play both JM and Busquets in midfield.
Because no top class defensive reinforcements means not only lapses in defence but it also means the likes of Xavi gets more tired and less rest.
It's not hard to figure out that you need Xavi fit and fresh, which in turns means playing and rotating JM and Busquets in midfield, which in turn means you need to buy defenders.
But hey I am not a Barca supporter, thus feel no emotional attachment, thus I don't see the need for "adding to the Magic" or spending £50m on Neymar when you have Villa, Pedro, Alexis and Cesc who can all play in the roles to support Messi.