Okay, a lot of people here apparently have only seen Cesc once he was here and didn't see him when he was with Arsenal so I am going to make a case in defense of this guy:
I think one of the biggest reasons why he has struggled particularly here is because he tried pushing him forward when in reality, we should have been pushing him back. The guy has a number 4 for a reason and I would like to propose the "false 4" if you will:
Soccer formations made easy with
this11.com
Cesc is a deep lying playmaker, he just doesn't realize it yet. He worked his magic at Arsenal by starting the play from deep in the midfield and pushing the team forward from there. His long passing was fantastic at Arsenal as he would make amazing 45-50 yard long balls over the top from very deep in the midfield. Those goals when you see him hitting 32-35 yard screamers (remember his beauty against Milan, then reigning CL champs?) came from him making bursting runs from deep into the attacking area. So I think he would best be used here as a false defending midfielder of sorts.
He has the grit and the tenacity to be a decent tackler, at least be able to disrupt the play. This is why I put Song in, as he will need another CM who can defend as well as surge forward, a true box-to-box midfielder. Song and Cesc linked up very well at Arsenal before and I think Barca would be wise to try to rekindle that connection they have. The remaining midfield spot should go to either Thiago or Iniesta imo and we should only run this if Xavi needs a rest.
But this would be a fresh take to our midfield game as we need to start looking beyond Xaviniesta and to how this team is going to evolve when they are gone and I think this could really work. Cesc flourishes offensively when he is the 2nd wave of attacks and the focal point in midfield and this system would work perfectly to exploit his strengths. He must be willing to do more of the dirty work defensively tho, but with that comes the freedom to attack whenever he sees fit. This could be a "Plan B" for us as well as it would completely disrupt how teams mark us in the midfield, add Messi in the mix and the DMs won't know who to pick-up. If anything, this will help make Cesc truly a complete midfielder who could do it all and as long as he is willing to play deeper and have more defensive responsibilities, I think he would really enjoy playing like this.
Just a thought, I know Tito would almost never even think of trying this but I think Cesc's problems have less to do with his deficiencies as player and more on him being miss-managed. Wenger gave him freedom to roam the midfield as long as he tracked back at Arsenal, and if Tito gave him the same sort of role here, he would finally have his own role at the club instead of trying to share a role with Messi and Xavi.
Go ahead Cesc haters, I know you will be up in arms with this one...
This is just a theory, I am not saying this right now would be better than Xavi-Busi-Iniesta, but so much of that relies on Xavi and since he is aging, we should start to play with alternatives. This should only be something that we try in La Liga games to see if it worked and let it develop before we even think of trying it in CL. But given time for Cesc-Song to start to rekindle their chemistry and I think it could work wonders.