Blizzard’s Lore provides:
Unlike Gul’dan’s death knights, modern death knights consist mainly of paladins who lost their faith and pledged their souls to the Lich King in exchange for the promise of immortality. Death knights who fall in battle are soon raised again to continue in their master’s service.
In the years since Arthas shattered the Frozen Throne and merged with the Lich King, the power and fury of the death knights has only grown. Now these unrelenting crusaders of the damned eagerly await the Lich King’s command to unleash their fury on Azeroth once again.
But wait. Once they reach the Outlands, which they must, right?, they’ll be doing quests for the Cenarian Expedition. Quests, like, oh, Save the Mushroom People, and Find My Lost Satchel. And Investigate Those Wierd Guys. Crusaders of the Damned helping out Aunty May and the Mushrooms?
Easy Solution: Make Northrend level 60-80. Now go ahead and add some 70-80 content to the Outlands. Provide more outlying Outlands that have drifted closer to the zones we have to provide more questing area. But also allow a questing path to take folks from the Plaguelands, 50-60, to Northrend, 60-80. This way the Death Knights never have to step foot off Azeroth and help the Mushroom People to advance.
And what’s this “Eagerly await the Lich King’s command”? ?! “Wait. … Wut? Slay all life? GTFO. Serious?”
How about this: All Death Knights, Horde or Alliance, can communicate, interact, trade, etc., with each other, and with their respective factions. Now that, my friends, would blow the pop top right off that can of brew served cold.