Blizzard has plans for the Hellfire Citadel info and the announcement of future five-person wing-dungeons in the Burning Crusade.  The Onyxia-type of instance sounds—and this is my pesonal guess—be a showdown with Magtheridon the pitlord.

Eyonix: “One of the first new dungeons that players will encounter in the Burning Crusade expansion is Hellfire Citadel, which is in-fact a five-person dungeon located in Outland. This dungeon will contain one really cool raid encounter (picture an Onyxia type set-up), but outside of that three wings will be fully dedicated and tuned for five players in their low level 60s.

As we’ve been stating since Blizzcon, our focus for expansion will be in creating winged dungeons that require smaller sized groups to complete. We feel players really enjoyed dungeons like Scarlet Monastery and Dire Maul and aspects of what made those dungeons cool are what we’re keeping in mind as we design our upcoming expansion dungeons.”