Arthas: Dashade is going to leave him alone for the most part. He'll hinder him here and there (as he did with the Plaguelands/Silverpine/Tirisfal repairs, as well as give Derenas a place to launch an attack against the encroaching Scourge with Fort Hillsbrad), but for the most part, Dashade is going to leave well enough alone. Obviously, the Alliance and Horde can handle their own (since they've done so just fine up until now).
Kil'Jaeden: If Kil'Jaeden is actually summoned into Azeroth, Dashade will do nothing but hinder him, as he does with Arthas. Dashade's given Marylinn "control" over Hyjal, and is going to be fixing Felwood over the next few months, if his word is true. Beyond that, he'll not directly confront Kil'Jaeden (nor Sargeras).
Horde Leaders: Not his problem - he's in the Rebellion.
Alliance Leaders: Not his problem - he's in the Rebellion.
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It's worth mentioning that most people haven't learned Dashade is an Eternal because he doesn't quite go around announcing it to NPCs.
It'd fuck with lore. I'm trying to avoid that as much as possible. It would be counter-productive to letting WoW evolve naturally.
And, despite the changes he's done, the changes have mostly "gone under the radar" due to the location of the changes (Fort Hillsbrad, Exodus, Tortuga, etc), or the inability to explain how said events occured (the Plaguelands, Silverpine and Tirisfal simply began to change back to their pre-plagued status - there was no large miracle to be seen like flying over sprinkling Eternal magic dust over the place).