it will probably be for the ToJ flashback/bran vision. they've already established flash backs in season 5, so ToJ is bound to come up at some point next season, especially with the r+l=j stuff, which seems to be all but confirmed at this point. they say he'll have two lines of dialogue, but he could be in a fight (like at ToJ) and have more screen time.
seems too old for arthur dayne. he'll probably be howland reed. or possibly georld hightower (LC of the kingsguard).
if he isn't in the ToJ flashback, he could also be randyl tarly. i can easily see him as randyl.