I don't know what could round more this team. I see quality on every position for the 3-5-2, not so much for the 4-3-3 since a lot of you don't seem to agree that that's the best formation for our team.
I'd bring on an striker of course, not really sure about who. Maybe try to get Kolarov again, he's one of my favorite players on that position, he's not as offensive as the position requires, but for sure he has a great touch on the ball and very good crossings, a potent shoot and good enough defensive covers. I wouldn't get another cb, rather wait for Masi to come back. And maybe just another midfielder, have you ever seen this guy Moussa Sissoko? He's a very talented midfielder, lots of stamina and a nice ball touch, current member of the France National Team; I'd say that he could rotate with Vidal when he needs to rest a bit. I'd let you a video of him with his skills, but I can't see anyone on YouTube. Anyways if you can watch a game of Toulouse I think that you'll agree with me, his style is of a box-to-box as Vidal and IMO he'd do great on Serie A.