I hope to see the blue wizards making an appearance in RoP and it would make some sense canonically because they arrived earlier than the other istari, but I don’t think the stranger will turn out to be one of them (where is the other one?).
However now that I think of it, is it stated somewhere that only five istari were send to middle earth? He could be the sixth, who fell fighting Sauron or a Dragon or Balrog for that matter 😉.

But I am afraid that you’re right and he’ll turn out to be Gandalf.