One thing I've been thinking lately:
He is seeing his team slowly but surely fall apart around him, seemingly getting a bit weaker every season (while their greatest rivals get stronger). If he stays at Barca but Real Madrid keep winning everything, he won't be winning the Ballon d'Or again. With Ronaldo almost certain to win the 2017 Ballon d'Or, that brings him level with Messi at 5 each. With Real Madrid looking like they will be the stronger team over the next few years, there is suddenly a distinct possibility that come the end of their careers Ronaldo will have won more than him, which significantly hurts Messi's legacy of GOAT. As such there is a possibility Messi may want to get out and start somewhere else so he won't be in direct competition with Ronaldo and the Real Madrid machine, which could boost his chances of winning more Ballon d'Ors as long as he leads his new team to the league title and minimum semi-final of CL.
Ultimately I think he will stay at Barca, but I am starting to wonder...