When we're playing 442, one of the main issues is the lack of bodies in the part of the pitch just ahead of the center circle.
With the 2 wing backs holding wide positions and the 2 wide midfielders ahead of them on the wings, Busi/Rakitic generally slot into the CB slot to get the ball moving, with the other one on the center circle to transition play. In this setup, Suarez is obviously upfront, and Messi drifts wherever as he sees fit. When I watch our games, I can see there's a clear lack of bodies in that central midfield because of this shape, due to which the most obvious pass generally goes sideways to Roberto / Alba, who have to initiate attacks with 1-2's with the winger or the central midfielder.
For a team that's parking the bus, attacks initiated this way are easy to defend because the play very rarely continues down the flank, and more often than not involves a cut back. This is the main reason, IMO, why chances are so hard to come by, and the reason why we see Messi dropping into that centre circle position.
Considering that Roberto doesn't provide THAT much in attack, let's be honest he isn't a Marcelo or Lahm, why not keep the full backs tucked in and play a 4-3-3?
If Roberto's starting position was tucked in and defensive, we could have triangles created between the full back, man on the centre circle and the attacker on either flank, with Messi providing the overload. This just seems to make a lot of sense especially considering our personnel. I can see why you'd want to play this 442 that Valverde does, which relies SO HEAVILY on our full backs (which is weird, because that's not even our strength), if you had Marcelo/Kimmich type players on there. As good as Alba is, he never really gets past players and his one move (although a very useful one so far) is the cutback to Messi.
It's never going to happen, because people will cry 'wing play is Barca's style, DNA, etc.". But considering that the full back position is one of our weaker ones on the pitch, it's very surprising to see how much of our attacking play is channelled through that position.