If Villa plays as a 9 they need 2 or 3 defenders but even that isn't enough to make Villa dissapear (unlike Messi) because he usually plays behind the defenders (aka offside). When he doesn't, he still can shoot from far away unlike Messi ie (2010 CL final style), or the goal against Alaves which was a powerful shot, plus his left foot is as good as his right foot, unlike Messi, therefore he can easily shoot anything that comes from both wings (Falcao style) without need to "set up" the ball to his best foot (like Messi does).
Then play Xavi, Dani Alves & Messi from the right wing
Then play Iniesta, Alba & Pedro from the left wing
It will be really hard to stop the right wing or left wing with all those insane "tiki taka" players. They will easily end up breaking any defense because Messi & Iniesta dribbling skills are awesome. Xavi, Iniesta can also just take advantage of Alba, Messi, Alves & Pedro speed and make them run.
Then we'll have Villa on the center, who is one of the hardest players to defend against, because of his play style. He doesn't need to get the ball to often, like Messi, as long as he gets a few balls, he will easily end up scoring because that's where he shines.
Let's not mention the fact that teams won't have a clue how to defend against us if we play this way... because we never have in the last years.
We should try different things. We've won a lot things in the past, but like I said, in the past. Just because we won things with a line up it doesn't mean it will make us win again. Just saying.
Just my humble opinion, but of course, one of the reasons why I'm writing this is because I'd like to see Villa play as a 9, and the fact that Messi usually ends up playing as a midfielder when he doesn't get the ball, and he could as well just play as a right winger instead & still be an attacking player and let Iniesta, Xavi, Busquets & Cesc take care of the midfield and keep doing what he does best: scoring goals & dribbling.
Another thing is that when teams park the bus we use our midfield too often (because Messi also ends up playing as a midfielder) and the game sometimes becomes boring to watch. Like I said before, I'd rather see Messi using his "dribbling skills" from the right wing, breaking up a defense in pieces, and scoring goals or assisting players.