Here's my take on it...
Defense :
Last couple of years, we have always been short in defenders in the most important periods of the year. Clearly we need to have more manpower there.
Seen the importance of bringing the ball in, I'd chose a defender with exactly those skills : Jan Vertonghen, young, experienced, not too expensive.
Back-up or replacement for Abidal : Either Alba or Gareth Bale. Bale would provide us with a 'left-footed Dani Alves'
Please play with 4 defenders, unless we're in a unusual situation (eg. second half yesterday)
Midfield :
Imo, we're ok here. Lots of quality for the 3 spots
Attack :
One of the main 'bad' characteristics of our play, is that we do not use the wings. I'd like us to get a young, yet somewhat experienced winger. Thinking of : Hazard, Neymar, Gotze, Muniain. My favourite would be Hazard, next Neymar. Gotze I'm worried about him being injury-prone, Muniain is maybe still a bit too unexperienced (not ready yet), though after one year at Barça I could see him grown to the type of player we need. I see too much doubt in guys like Tello/Cuenca, even Pedro, to take up their man.
Messi dropping too deep, meaning we don't have a reference in attack. Imo, Sanchez is exactly what we need at CF, his runs remind me of Eto'o, going behind the defense, great speed, great fighting spirit... This means either Messi going back to right-wing (where he was taken away in order to not tire himself pressuring the opponents), or slightly adjusting our system (messi as a number 10). Although I never was in favour of changing the 433, having Messi in your team might be a good reason to do so.
General :
I'd like to see us loaning out the Cuenca-Tello type of players, so they get experience before playing at Barça. In games like these of the last week, you need experienced/proven players (unless you win, because then you're always right).
I'd like to see us using more of a sharp counter-attack, if the situation is there to take advantage of. We always seem so slow in building up, whilst at times there's like an ocean of space to explore after regaining possession (learn some from Madrid in this sense).
I still believe in this team, I still believe in our philosophy, the team, the coaches... Yet sometimes to keep your level, you should improve in some points. Learn from your mistakes, try thing differently. I don't want to change everything, just keep what's good & proven (last 4 years is good, & proven), just make some slight changes, you can always improve.