It might sound weird, but I'd play a 3-5-2 when Neymar comes.
----------Neymar/Tello----------Messi/Deloufe-----------
Alba/Sanchez---Iniesta/Cesc--Thiago/Xavi------Alves/Pedro
---------------------Busquets/Song------------------------
------Adriano---Pique/Masch----new CB/defensive RB----------
The 'wingers' could be more defense minded like Alba and Alves, or play Sanchez and Pedro or a combination. Adriano can play both wings and also as defender.
One of Thiago, Cesc or Iniesta could be more attack-minded in this system.
Neymar and Messi each have their 'half' up front and could alternate to help out in driving the ball forward from midfield. Sanchez can also play upfront.
It simply would give us more options and makes us a little more unpredictable. Both Messi and Neymar could alternate as false '9' and also stretch the defense or go out wide sometime - this would open up for Cesc/Thiago/Iniesta in the AM spot. Wingers could play almost as in the system we play today, also alternating being very offense minded, or like against Bayern or other strong teams, stay back a little.
Our DM will fill in for the attack minded AM when in control of the ball since we play with 3 CB's. And Pique could also drive the ball forward (which he does very good) without risking much.
Neymar will push a little more forward than Messi so we actually have somebody in the box from time to time.
But, we could also need one more forward in this system since we can't rely on Pedro to score (but maybe in this system, who knows), Villa is out and Deloufe is far from ready.