The update to The Waiting Game revealed that Dhuum will appear once all of the Underworld quests have been completed, and defeating him is now required to "complete" the Underworld. With a skill set including things like "Judgement of Dhuum", this new addition to the Underworld is most likely the challenge that bored Guild Wars players have been looking for.
Shadow Form continues to be a hot topic, however. Many players expected this to be the thing that put an end to Shadow Form Underworld farming, but one of the earliest reported successes proved that wrong very quickly. Players responded quickly with one commenter summing up many people's thoughts: "It's...disappointing that even a God of Unholy Death can't bypass shadowform...". (Community Manager Regina Buenaobra stepped in to mention that the goal was new content, not a nerf of SF farming.)
In the meantime, Dhuum seems to be the biggest challenge around in Guild Wars -- kudos to ArenaNet for the new content!