Anything to do with Death Knights!
With the huge amount of death knights being created, there will be an increased demand for death knight inscriptions, Bind on Equip plate gear that is better than their starting epic blues they get from quests, and any consumables/enchants/gems they can use to speed through Outlands. There’s much profit to be made here, but make sure you base them all around what a DK would want and not other classes, as they are all in WOTLK.