Thematically: The dracthyr would be created specifically to prevent threats to the timelines in dire need. He could have told no one and erased his own memory of them so that they could eventually be used against Murozond, because he knows the only way to defeat him is with something he doesn't already know about.
Customizationally: They'd have a reason to have all races as visage forms. They're sleeper agents.
Timewalking Campaigns: They'd be the ultimate canon retcon. Murozond succeeds in a significant step towards shattering the timeline, and then the Dracthyr awaken. Across all of history. So they were always there. Questgivers in older expansions are exposed as Dracthyr, but in such a way that it's as if they always were. Dracthyr get their own subfaction within the Alliance and Horde that have been established since the First War, there were Dracthyr that died on Teldrassil and in the fall of Silvermoon. Dracthyr Death Knights.
Just thoughts.