An additional point of note is that the Horde was already home to an unscrupulous-type, Forsaken, which would've provided a platform for the Void Elves to thrive within the narrative (meaning, it'd be easy to tether Void Elves and Forsaken together as a "voting bloc" for the Horde); similarly, the Alliance is already home to a group of egregiously arrogant knife-ears, Highborne, who could've easily been utilized as a narrative springboard for the Nightborne insofar as internal politics go.
I would argue that Blizzard's internal dilemma regarding implementing playable High Elves has always been more about not disenfranchising the sizable portion of the playerbase that currently play as Blood Elves. If their primary goal was appeasing the fans of High Elves, then they'd just implement playable HE's and respond to the cries from the Horde with gems like, "you think you care, but you don't".
That is set of priorities that I, personally, can absolutely agree with. I feel like regardless of any other opinions on the subject of Blood Elves, generally, we can all agree that since they're a playable faction already (and theoretical playable High Elves aren't) they shouldn't have to worry about losing out on things that were developments to their particular narrative(s), so that Blizzard can "make room" for additional races.
Agreed. It would solve this particular issue, though.
I'm curious about your thoughts on how the game might function if Alliance and Horde continued to exist, as plot devices to drive narrative development, but as much less centralized organizations and largely co-operative with each other?
You might expect to see both Horde and Alliance being represented at most/all major conflicts, each with a different perspective and approach to the issue of the day, but ultimately this doesn't affect who we (the players) can and cannot form a party/raid/guild with. I would maintain that the races remain attached to their faction, as opposed to being "free agents", but be capable of cordial relations with people from the other faction.
They have flourished, this is true.
I'm more concerned with them being culturally asynchronous with the member-states of their respective factions. It also doesn't seem extremely far-fetched that the Night Elves would be capable of forgiving an aggression, when doing so results in them rekindling an ancient friendship (Tauren) and aligning themselves with two new races whose mannerisms mirror their own (Orcs, Trolls).
That scenario doesn't present as being any more far-fetched than Blood Elves forgiving an aggression (also perpetrated by Orcs), when doing so results in attaching themselves to the remainder of the group which annihilated their nation (Forsaken), a group which invaded and slaughtered thousands of their outlaying citizenry (Orcs), and a group that was at the time indistinguishable from their only ancestral enemies (Trolls).
I think I would've just started the game with the Night Elves on the Horde and the Forsaken on the Alliance. This followed by High Elves for the Alliance, and Ogres for the Horde, in TBC. Everything else can remain the same. The obvious problem with this being there wouldn't be any Draenei. Though, both WoD and Legion might've been adequate times to release them (as they got more story in WoD than they had in all of the content from TBC through MoP).