Blizzard has put up a preview of the first of the three five-mans coming in 4.3 - End Time. What bosses do you fight? Sylvanas, Tyrande, Jaina and Baine! The final boss is Murozond, a mysterious dragon with a very familiar name.

» SourcePatch 4.3 Dungeons Preview, Part One: End Time
One of an infinite number of potential outcomes, this timeway depicts the desolate future of Azeroth should its defenders fail to stop Deathwing. In this bleak future, Nozdormu has identified an anomaly that bars access to both the past and the Dragon Soul: a powerful creature from out of time, living alone amid time-twisted echoes of the past.
In order for Nozdormu to provide you the ability to travel back in time to a point before the Dragon Soul was hidden by Malfurion Stormrage from anyone who’d seek its power, you must first go to a distant and desolate future to discover the anomaly blocking the past. You'll find yourself in a very different Dragonblight, stripped of its snowy landscape, with little more than ruins of the once-great dragonshrines. Wyrmrest Temple itself stands only as a reminder of Deathwing's madness -- what's left of his scorched remains is impaled atop the structure.

Before you confront the creature disrupting Nozdormu's access to the past, you'll first be called to fight any two of the following four ostensibly familiar leaders, chosen at random each time you play through End Time.

Links
Battle.Net Blog - Patch 4.3 Dungeons Preview, Part One: End Time