Sergi Samper is like 18. Doubt he'll be good enough to replace Xavi by the time he (Xavi) retires. Plus, he won't be in the first team for a while, and we need a backup for Xavi right away if Thiago leaves, since Xavi will need to be rested often due to his age. Plus, isn't he more of a CDM than a Xavi-type midfielder? And Isco's more of a CAM/Iniesta type, which we already have 2 of (Iniesta and Cesc). As much as I'd love him at Barca, we don't need him.
IF Thiago leaves (God, I hope not), the guy we need to get is Gundogan. Hell, he's the only player I can think of that can replace Thiago.