Yeah you have a point, but as usual it’d be more like a skirmish that happened because both sides had a misconception about what the other is doing. You could just copy Ashran and have both sides find out about an ancient Elven artifact buried in the center of Silvermoon which is still in ruins and controlled by undead. Both sides launch an attack to claim the artifact as both sides think they have a rightful claim. Technically the expedition is about fighting Scourge, but Horde and Alliance run into each other and the fighting begins.
This wouldn’t be the first time both factions erupted into a large conflict over ancient artifacts during peace time, Wintergrasp and Ashran were both such instances. And it’s barely touched upon outside of the zone conflict itself, and the main big bad of the expac is defeated anyways.
And the High Elves have shown time after time that their population may be smaller, but they’re able to counter the Blood Elves wherever they go. And with the Void Elves on their side, Blizz opened an avenue for them to have just enough Void Elves for any situation since they recruit from both Alliance High Elves and Horde Blood Elf defectors, you see both in Telgorus Rift.