Blizzard have announced that the next Timewalking raid they're releasing is going to be Ulduar. Wowhead posted this:
About Ulduar
Ulduar, the prison of Yogg-Saron, was a sprawling raid in Wrath of the Lich King. It had an innovative way to start hardmodes--instead of switching the difficulty of the encounter before fighting, players would have to start the encounter in a certain way to unlock the hardmode. For example: defeating Thorim's gauntlet in 3 minutes or less would trigger his hardmode.
There were two end-bosses in Ulduar that awarded Feats of Strength and titles: Algalon the Observer which was only open an hour a week, and Yogg-Saron which had multiple hardmode steps.
Currently, Ulduar is worth farming for 
Mimiron's Head, 
Val'anyr, Hammer of Ancient Kings, 
Vanquished Clutches of Yogg-Saron, achievements like 
Glory of the Ulduar Raider (25 player), and some great transmog gear.
Ulduar is a fitting Timewalking raid leading up to Battle for Azeroth with all of the recent Black Empire and Old Gods speculation.
For the short version of clearing Ulduar, check out Henderman's Burn 'em Down: A Pocket Guide to Ulduar and for the longer version covering all achievements, please read eidyia's Ulduar - The Complete Achiever's Guide.
