I think there'll be a quest reward which you get for completing the raid which you hand in at the tower for a class item or a piece of exotic loot. And I think the bosses involved in the raid will probably have the chance to drop a piece of an exotic loot set. I think it would be really cool if a new subclass was the reward though. Complete the most difficult challenge the game has to offer and then they throw you a new class to grind completion of. It would certainly add another bit of replayability.