Like if we go off the actual lore Warlocks are more powerful then many of the Horde and Alliance leaders with the potential to become even more powerful at the risk of becoming insane.
Like whats stopping us from enslaving one of the Nathrezim in Zereth Mortis and using them to say replace Anduin and take over the Alliance and destroy them from within like Balnazzar did to the Scarlet Crusade?
The Uncrowned, probably.
They already stopped a dreadlord from trying to infiltrate the Alliance at the high levels once before.
It turns out no matter how powerful you are, nobody is immune to a rogue figuring out what you’re up to and assassinating you.
Well doing that would probably work out even better if we tracked down Anduin and had him possessed they’d end up assassinating their own king
Warlocks by nature are self serving. You’ll cook up a plan, and end up either backstabbing each other in the end or exploding via fel microwaves in the attempt.
They’ll form ranks, sure, but they can never reach the level of organization as, say, the Ebon Blade.
They’d just assassinate the warlocks and any dreadlords working with you.
Then figure out a way to get him de-possessed.
Retirement plans?
Simple Solution set up a fortress in the Twisting Nether on an old legion portal world guarded by armies of powerful demons sure to achieve this will probably require becoming so powerful I go insane and probably at least partially become a demon but the power
Well now you’re basically trying to reform the Legion, which I guess puts the Demon Hunters back into a job.
Or we just send all the leaders nobody likes at you. Either they beat you and we win, or you kill them and we still win anyway.
I am once again petitioning for the Black Harvest to summon forth Legion spacecraft, enslave their crew and scourge the stars as pirates.
Because they are my friends and won’t leave me. c: 
Warlocks do not have friends we just pretend too you’re all really just pawn in our plans for ultimate power
That’s fine, as long as we do it together. 
I don’t serve anyone but myself. This isn’t even my original body. I am whoever and whater I need to be.
The legion was formed to defeat the old gods my goal is to destroy all that have wronged my People however sending in an Army of demons is too risky better idea to instead take over the Alliance and use them to wipe out Sethrak permanently before destroying the Alliance from within and the Horde shall be spared to clear my Peoples life debt to them after saving them
I vote we start by assaulting the felhammer and taking back meta.