Lavezzi would be great but cannot see him leaving Napoli in Jan, despite them signing a possible replacement for him recently. As a direct replacement for Tevez then I'd love us to sign Cavani (I'm still not too sure that Dzeko will become a great target man for us. I think we need someone like Drogba was in his pomp, and Cavani has looked superb in the few games I've seen him in this season.)
If we can go for players like Hazard and Reus then we should have enough attacking midfielders/wingers in our squad to have a full assault on the Champs League next season. I'd love us to sign Muller though.
I hope we do sign a centre back in Jan, Kolo will be off for a month, Savic is learning fast and hopefully will live up to his potential as the next "Vidic". We only really have Kompany and Lescott with Savic as a young back-up. I'd like to see us sign Hummels, but maybe a pre-contract for the summer sorted now. Alex seems a good shout as a quick fix for a few months, perhaps ?
As for midfield, De Rossi keeps being linked, Javi Martinez also seems like a long term replacement for Barry. It would be great if we could sign someone like Schweinsteiger though, but he will be a one club player.