Well, if Aria ends up being "A" (see below), it would only make sense that Ezra's in on the action too.
We know Ali was dating an older guy, to whom she would only refer to as "Board Shorts." Ezra obviously likes younger girls, because he's dating Aria. Can you see the connection?
Ezra seems pretty pissed when he finds out that the Ravenswood lair belonging to "A" has been infiltrated.
Ezra is desperate to get the diary Hanna stole from said Ravenswood lair — so much so that he ransacks Hanna's room looking for it.
Remember that sketchy drawer of money Ezra has in his apartment? We never really got the full story on that.
Ezra coincidentally shows up in Rosewood right after Ali's disappearance.
Why It's Not Possible:
We've been tricked before. If Ezra really seems like "A" (which he does), it could be just to throw us off.
He's reckless! He yells and smacks cars in the middle of Rosewood, rifles through Hanna's room with her mom downstairs, and listens to recordings of Ali in his classroom. "A" is a sneaky mastermind and never gets caught.