The thing is we are not trying to fill the hole of not having a midfielder, we are trying fill the hole of not having a WC midfielder. To me, this transfer window is a failure if we just buy a decent player for CM, even if we got a good RB. We need quality. I would trade not buying a RB, not buying any other player at all... if it entails us getting the best midfielder out there. It cannot be taken lightly.
This is the key to this offseason. The quality of the first 11 midfielders Iniesta and Rak was not good enough last year, Barca got outclassed in midfield way too often. Iniesta was available/good enough for less than half of last season. He is not likely to ever again produce full seasons like he used to.
We need to add one of the currently best midfielders in the world, so that either Iniesta or Rak can be high-quality depth. No matter what formation is used going forwards, the chances of winning either Liga or CL without adding a fantastic midfielder are not good. Or RW in 4-2-3-1.
Unless the team gives up using a 4 man backline in the more challenging games, an upgrade at RB is needed. This does not have to be one of the best players in the world, though that would certainly help. If the team converts to 3 in the back for the bigger games, then instead of a RB, another CB is needed for depth (assuming Mathieu leaves and Marlon is promoted).
The next biggest need is a #4 forward who can add more than Paco. Preferable some on who could play on either the left or right.