Rise of iron: not all characters are unlocked to play the new expansion

EDIT: With the announcement of the Taken King DLC, a new subclass for each class has been revealed, coming this September:

• Titan: Sunbreaker (solar)
• Warlock: Stormcaller (arc)
• Hunter: Nightstalker (void)

Note: The following information was true upon original release of the game (pre June 2015), but new subclasses have since been announced.

This slot is due to the way subclasses were supposed to work. The mechanic I'll describe below was removed on the day one patch:

Removed subclass locking from builds.

Basically, the way subclasses were supposed to work was as follows:

1. You level up a subclass all the way, choosing which perks you wanted.

2. The final ability "locked" the subclass (can't change perks anymore) and gave you a slight boost to stats

3. Now that you've locked this subclass, you can start leveling a new subclass (either the same or your other option) with an exp bonus to level it faster

If that wasn't clear enough, see this article

The point of this was to experiment with different builds. They removed this feature in favor of freely swapping between subclasses. It seems it was an oversight to leave the third subclass slot in the character menu.

The engram will decrypt into another exotic item, not the same item. However, you may get an item for the hunter. There is only a very small chance that it will be the same item that you received from decrypting the exotic engram you already bought.