In a word: Neymar.
Next season—perhaps more so than any in the past five—will be a major transition year for the Barcelona squad. Aging veterans and former fixtures at the Camp Nou—Puyol and Xavi—will see less time in the first team, while Neymar and—most likely—a new central defender will be thrust into the limelight and expected to hit the ground running. And with Thiago seemingly headed to Manchester, Barcelona's midfield may be a significantly weakened outfit over 2012-13.
But Barcelona's biggest problems last season, the defects that kept them from reaching the heights that supporters have come to expect, were in defense and in front of goal. In particular, as the season wore on, it became clear that Pedro and Alexis couldn't find the right goalscoring touch, and Villa was unlikely to fit into the first team. Messi, therefore, was the only consistent goalscoring threat.
In key games—those at Milan and Munich, to name a couple—opponents were able to mark Messi out of the match and neutralize Barca's attack in the process.
With Neymar alongside Messi, opponents will no longer have that option.
Pedro and Alexis Sanchez, as dutiful and hard-working as they are, failed to provide a counterweight to Messi's threat down the center and were generally ineffective as alternatives for final passes from Xavi, Iniesta or Messi himself. They were decent providers (contributing five and 11 assists, respectively), but Barcelona needed more out of them and didn't get it.
Neymar has the finishing touch to match his creative flair. If his Golden Ball-winning performance in the Confederations Cup is anything to judge by, he has all the tools necessary to fit seamlessly into the Barcelona attack, while providing the goals that Barca have been missing.
With a hefty price tag and such high expectations, there is always the danger that Neymar will have difficulty adjusting to the increased pace of the European game, the pressure of life at the Camp Nou or both.
But with a serious dose of confidence to match his massive talent, the little Brazilian is unlikely to flounder as other big-money transfers before him. And perhaps as importantly, Neymar has plenty of playing time under his belt at the club and international level. He makes the move to Europe not as a naive youngster and untested prospect, but as an already experienced player.
Overall, Neymar will add serious bite to an already dangerous attack. With Neymar alongside Messi, anything is possible for the Blaugrana this season.