Winter is Coming reports that Tom Wlaschiha's agency in Germany has him listed as filming on Game of Thrones
starting in August. If that's correct, Jaqen will make his return next season. Even before this report there was a lot of speculation whether he'll be the one to train Arya in Braavos. My guess is he will be, perhaps with Arya meeting the Kindly Man and him changing his face to reveal himself as Jaqen relatively quickly.