Last night, Blizzard blue-poster Zarhym made an announcement on the WoW forums confirming the introduction of a brand new five man dungeon in a future patch. Whether or not this instance will make it into the next large content addition is uncertain, but its likely to be sooner rather than later (I hope?). Zarhym stated that the new dungeon would be similar to Magister's Terrace, which was a dungeon released in patch 2.4 during the Burning Crusade days that dropped gear on par with that from Tier 4 raids of the time, like Karazhan. At the time, Magister's Terrace was considered the most difficult 5-man in the game, and our new dungeon will likely follow the trend.
If our successor to MT is going to be able to claim to be a true group-breaker, what has to happen? The current status-quo on Wrath dungeons has been that trash mob encounters are simply AOE fests, far different in style from most dungeons in BC, where planned crowd control and carefully timed pulls were usually a necessity. Blizzard seems to be emphasizing that they want more focus to be put into the creation of elaborate and boss encounters, rather than having you palm your face over and over as the PUG you are running Underbog with wipes on trash for the fifth time. However, plenty of players voiced their dissatisfaction over the ease at which trash mobs could be downed in both 10 and 25 man Naxxramas raids, as well as the various dungeons throughout Northrend.
With the release of Ulduar, a large difficulty jump in end-game raiding has taken place. Trash mobs are again very capable and willing to kill you. Its my guess that our child of Magister's Terrace will contain a combination of challenging in between fights as well as complex and inventive bosses. Or, we could just end up with Heroic Wailing Caverns...