My first choice would be a dominating skilled midfielder to play with Sissoko/Marchisio. I think that hole is even more important than the winger hole, since I'd have faith in Giovinco and De Ceglie as my left wingers. Though I'd also try to snag some depth like Mesto and Rosina to replace Marchionni and Salihamidzic and be a backup for Camoranesi, and in Mesto's case, depth for the RB position too.
My shortlist would be:
Iniesta
Gerrard
Xabi Alonso
Diego