I mean, the internal strife doesn’t even have to be that pronounced. Work with the foundation we already have. We saw how the Dwarves had been paralyzed in MoP because neither the Ironforge or Wildhammers felt comfortable leaving Ironforge for fear the Dark Irons would try to take it again.
Worgen have barely had the ill-effects of their curse elaborated on since they left their own starting zone. Even in Duskwood, the most we saw was them avoiding Darkshire.
These sentiments of mistrust, of suspicion, should not be something hand-waved away so easily. These should be lingering sentiments that never truly disappear. All these, ‘Dark,’ races in the Alliance should be banding together not just because of common ground, but because of a sense of mutual support and safety. Literally, an Alliance within the Alliance, existing to defend itself.
From there, let the story evolve naturally. This, ‘Dark Alliance,’ embraces a more pragmatic approach. What we saw with the Void Elves in BFA is perfect for this (one of the few times I’ll praise what little writing they have). Void Elves in BFA committed atrocities of a magnitude that cannot be ignored, but are largely given a pass because of their justifications. They throw Horde diplomats into the void to die of starvaton, thirst, or whatever is lurking there, to deprive the Zandalari of information, under the guise of, ‘it’ll save more lives in the long run.’ They reanimate a T-Rex to wage psychological warfare against the Zandalari to break their moral and reduce casualties on both sides.
This is war crime level stuff, but it gets justified in such a way that even the holiest of Paladins would turn a blind eye to because it could be so much worse (and those holiest paladins do, our Lightforged buddies who seem to get along oddly well with the Void Elves of all people).
If that sort of approach was taken with this, ‘Dark Alliance,’ where we see pragmatic but downright evil methods being used to accomplish goals, it opens up an internal conflict in the Alliance. What’s more, have this, ‘Dark Alliance,’ actually making progress where more traditional, moral parties in the Alliance don’t. Build this idea that, ‘if we want to survive, we need to be willing to do what is necessary.’ Start this cold civil war off on ideology.
I would sooner accept Anduin as someone who acknowledges the darkness needs the light and vice versa, if he can bring these two groups together after all that, and accept for himself that sometimes, bad things need to happen for good reasons.