I used broad strokes to make my point just as there are those that use broad strokes when trying to argue against High Elves when we all know it's a bit more nuanced than that. If one is going to reduce High Elves to broad strokes, I don't see why the same cannot be done for all other Allied Races. Again, there are varying degrees of similarities as differences with each of the Allied Races that already exist. This is increased similarity that High Elves share with Blood Elves is similar to the differences between Highmountain and Mulgore Tauren. Or even the potential Wildhammer Dwarves, which we know devs also excitedly discuss.
Nightborne are the Highborne portion of the Night Elves. They are the Night Elf counterpart of an Allied Race. You can reduce the "purple elves" bit all you want but there is a literal quest in-game making reference to this that I've linked many times, I'll do it one more time though I do not believe you haven't already seen it:
https://www.wowhead.com/quest=49427/...r-purple-elves
It's implying the connection there between Night Elves, which the Nightborne do have whether you'd like to downplay that or not. Again it is all about varying degrees of similarities and differences, that's my main point.
Nightborne are still another form of "arcane/magic focused elf" they just bring that same concept in through a different view. This was in reference to you implying that a unique combination of already existing traits does not make something unique, when it clearly does and Nightborne are an example of that. You can look back over to discussions when people felt the Nightborne should have gone to Alliance over Horde and you will see many pointing out the similarities between the Nightborne and Blood Elves. Being another arcane focused elf did not stop them from becoming an Allied Race.
Again, varying degrees of difference. You say there is no difference but we already see differences: Paler skin/hair colors, different npc voice chatter, no eye glow. That's not even including that when a race is made playable they get extra customization on top of that (this point also keeps being downplayed, but it is a fact).
Their story in WoW has shown to taken a different turn than what the Blood Elves have gone through. In TBC they weren't being brain washed by their own society to accept their "new faction" (because they've never left the Alliance in the first place, even when their leaders asked them to), we also saw reference in-game to how High Elves regarded Blood Elves ("Don't worry, I'm not a Blood Elf, I won't suck the magic out of you"), and Wrath introduced the human/high elf city of Dalaran where we saw the largest gathering of High Elves. Cata we see Halduron ask Vereesa to help because he holds her fighters in high regard, even against Lor'themar's wishes. MoP exploded the rivalry between High Elves and Blood Elves into a bloody purge. Legion showed us the overall culture of each of the three elven groups: Blood Elves, High Elves, and Night Elves.
Their story leaves room for more for sure, but they already have a lot to work with, certainly much more than races created out of thin air (Void Elves), and similar to other races that haven't done anything major for a long while but got added (Dark Iron Dwarves).
No I was saying it's splitting hairs to say that the AR system is to bring about unique options, and then say only thing High Elves would bring is a "unique combination of things that exist", because that by definition would be unique and the Nightborne are a prime example of this. Let's take each of your Nightborne points and test High Elves against this:
Unique model: Already pointed out differences between them and Blood Elves, this also isn't taking into consideration extra customization that would be added on top once becoming playable.
Again, the difference between HMT and Mulgore Tauren is Antlers vs Horns, and Tattoos. Dark Iron Dwarves did not have fiery-wicked beards nor tattoos before becoming playable. You can't be someone that is okay with those additions/variations and then say High Elves must strictly stay the same way they are. Even Nightborne got additional hair colors once they became playable.
The model is the easiest thing to change around.
Fantasy: This one High Elves have in spades, they're not haughty, they're not proud and they're not xenophobic. As well as being the minority of their people and choosing loyalty to friends over family, they represent a different moral choice. That's very compelling and this isn't expressed by any current playable race.
Architecture: High Elves have been shown to have architecture similar to Blood Elves with a blue paint job instead, I'll agree there. But they've also been relegated to Lodges as well; lodges that look more Night Elf and clearly not Blood Elf. This is another "easier thing" Blizzard can emphasize/change: the use of lodges for High Elf architecture.
Story: This is another easy one, the High Elf story on Alliance following the split of High Elves and Blood Elves has been different. We already know from Chronicles that even before the split of identities (Blood Elf vs High Elf) there was already a different cultural/personality mindset between those High Elves who lived in human lands/kingdoms (like Dalaran) and those High Elves who stayed within the walls of Quel'thalas. That's strong seeding right there. It's been emphasized through WoW and just needs more brushing up if they become implemented as an Allied Race.
Cmon, you know this is disingenuous. When people say they want to play High Elves they are specifically talking about that current group of elves on Alliance that still identify themselves as High Elves today.
Do Blood Elves continue to identify themselves as High Elves? No they do not.
It's not about playing a race, if that were the case we would not be having 2nd versions of existing races like another Tauren, another Dwarf, another Draenei, etc.
Also, as from before, we know that Wildhammer Dwarves are something devs know about and discuss cool ideas about. Therefore we know that this player-manufactured "there won't be a 3rd race" isn't true until we see how they deal with Wildhammer Dwarves. Or High Elves for that matter since Ion told off-mic that High Elves not being in BFA doesn't mean they'll never happen.