Which club will Messi play for if he leaves Barca?

Lionel Messi telling Barcelona that he wants to leave after spending 20 years at the Catalan giants (Messi joined the Barcelona youth academy at the age of 13) sent shockwaves across the football fraternity. Though every effort is being made to change his mind by Barcelona officials, if Messi decides to go ahead with his decision to leave the club, he will then play for a new club some time or the other.