This may sound tactically insane, and it will definitely be insanely expensive to implement. But adding another world class forward like Reus into our XI could be a game changer for us, not necessarily because of the added firepower, but because it would be ideal for Messi. With Reus on the right, Messi would get to play in the center behind a lethal CF with a tremendous workrate. Which means more assists since he'll have more opportunities to combine with RSN, and probably more goals because even though he will be playing deeper it will be easier for him to capitalize on the pockets of space created by RSN.
It's risky, but I think we can pull it off if we can find the right CM for a double pivot (e.g. pre-injury Gundogan). It would also add a lot of depth, because if one of our forwards gets injured we can always bring Iniesta as his sub.
For RB, I don't see why we can't play Mascherano in that position. We've always played with an extra CB under Pep (Abidal). It's a position more suited to him than either CB or Barca DM, and with someone like Reus on the right, I don't think we will need to worry about his offensive contribution.