Regardless of what you think, the current alliance between those two, is because of hte 2 faction system that has races that don’t fit the horde in it.
If you were to remove that, you have no real relevance for the Nightborne to be with teh blood elves, it serves no function or purpose, the Blood elves, Void elves, San’layn, Fel Illidari Thalassians, Farstriders, Sunsworn, Magisters etc - they are a complete package - Nightborne don’t belong there.
If htey were reorganising the factions. Humans, dwarves, (high elves), gnomes, worgen gilneans would be alliance, with orcs, tauren, goblins and trolls horde.
Night elves along with Nightborne, and Worgen Nelves, with Draenei would be a faction.
Now, I see no reason why highborne and Nightborne wouldn’t have friendly relations with Thalassians, high elves or blood elves. There is also avenues for void elves to want to be friendly with night elven black moon priests, highborne/nightborne esp void knowledgeable SHen’dralar and avenues open up.
There is also room for friction, disagremeents of ideologies between high elves and blood elves - not enough to make them kill each other any more, that antagonism was created in the two faction system, out of it, even with high elves being the human supporting wing of the Thalassians, there would be no need for killing levels of conflict any longer… however friction and love can exist - love from reconciliation - which can be quite healing, but also frction for those very hurt by the past and who strongly opposed to the different ideologies.
Same with Thalassians and Darnassians, again, out right hate would not be necessary like it was when they were locked to two factions that had to hate each other, but there can be an element of it, but it can be nuanced, some hate, some love depending on groups. Many Thalassians hating druid darnassians, but some exceptions can exist - like void elves consulted to help protect he emerald dream from the void … yet priestess order split, some storngly in support of bringing all elves to Elune, others just had it with any off spawn of the highborne - despite being allied to HIghborne - but they trust their Highborne they know are pure… etc.
Nuance…
This is just one example.