1) In content updates, we'll see more dungeons added to this.
2) Dungeons might be related to old lore and old GW missions. For example, Underworld and Fissure of Woe are in the Mists.
3) Possibly tied in with the up coming pre-cursor changes. I.E if you get to a certain level of difficulty / collect enough tokens you get awarded a pre-cursor / pre-cursor token. Likewise, possibly drops from the 'special bosses'.
Edited by Zedabi, 12 November 2012 - 04:21 PM.