Random? It was put purposefully into the end of the expansion to foreshadow a future expansion. BFA sets a LOT of stuff on Azeroth that isn't cancelled just because Shadowlands happened.
Dragon Isles get one mention: Scarlets get three(?). Far more concrete foreshadowing (this group is coming back) than the crazy fish Tauren and Kul Tirans/Zandalari in Legion pointing to BFA.
Okay, let's go with this for a moment; The new expansion location is Lordaeron, possibly the entirety of the Northern half of the Eastern Kingdoms;
What is the goal/point of this expansion? Are we moving the Forsaken out of Trisfall Glades? Are we restoring Lordaeron to becoming a Human kingdom again? Are we fighting the Scourge? What are we doing exactly? Blizzard said we're done with faction conflicts, so us removing the Forsaken seems unlikely. Are we seriously going to push the Scarlet Crusade as an antagonistic force that can fight off the Horde AND the Alliance?
Also, modern WoW expansions don't stay in one location. Where else are we going? We can't mope around Lordareon for an entire expansion.
Finally, and most importantly, if I'm an Orc/Troll/Tauren/Goblin Horde player, or an NE/Draenei/Gnome/Dwarf/Worgen Alliance player, why would/should this interest me?
They added in the weapons and mount because no work was required, the items only exist. WoW races are not a meritocracy that get more shit based on how many people play them: Humans and Belfs have got fuck all for a long time.
The Velf weapons also exist so they will be added in.
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Scarlets + MU Army of Light + AU Lightbound. Danuser was on record saying Yrel would play a part in the future, and then you have the potential of exploring "Lightlands" and whatever creatures come from there to mess with Azeroth.
Sylvanas still suffered and lacked a will of her own until the Throne was cracked. She still had to contend with being a tool against her own people and lacking her autonomy. The Forsaken died to the Scourge just like she did and pre-Cata were also tools before finding free will.
Calia was completely safe away from Lordaeron while everything happened. From the time she was killed to her revival was extremely brief and the circumstances of her revival are entirely different and it is presented as far less of a "curse" or "torment."
They are thematically far removed from one another. The idea of it is patronizing. Like a trust fund baby teaching the poor unwashed masses how to live.
Nope, that is exactly how it works, and that's why Humans and Blood elves get lore developments every expansion. That is also how other (more) popular games work, see for example how in LoL only the most popular champions get the legendary and ultimate skins.
That the Void elves haven't got anything new in so long is simply because Blizzard hates them...
Why would the Army of the Light want to attack the Alliance or the Horde? Their sole enemy is the cosmic void. Also, why would those forces even work together in the first place? The Scarlet Brotherhood would view Yrel as an alien threat just like anyone else would. Further, the Lightbound are not genocidal maniacs, they offer non-members the opportunity to convert. The Scarlets just slaughter the scourge and forsaken indiscriminately. Yrel's forces would not support that.
What single development has the human race seen since Cataclysm?
Anduin =/= humans. Belves have gotten more than them with Legion/BFA/SL advancing their story but there have been absolutely no developments to the Human story beyond the Wrynns since the heelturn of Benedictus... that humans don't even know about in-game.
Just like every other cosmic force, they want to claim Azeroth to turn her into a Light-Titan to conquer other forces and/or use Azerite to power whatever cosmic MacGuffin they are using to Lightforge all of reality.
Plus Azeroth's denizens are diverse and chaotic. They don't adhere to Light's One True Truth -worldview.
New plot developments. Xe'ra corruption. We already know that the Scarlets have reformed in a "different way" with Calia's child being a prominent figure and/or captive, so clearly something is different about them. They will try to wipe out Calia and the Undead again but surely they would have something more to them.
Army of Light goes with Turalyon, so if Turalyon sides with Xe'ra/Bad Light over Alliance and Horde they go with him. Except for Fareeya(?) who will be "one of the good ones" to show not all of them agree with that idea and so LFD's can still be a playable race.
Yrel's Lightbound has that converting aspect to them so she could be a somewhat more sympathetic force than Scarlets (easy) and Turalyon (if he goes cuckoo).
These three forces being different doesn't go against them working together. If anything it just proves that the Light could make for an interesting antagonist as there are three very distinct groups with different ideologies and functions for us to fight, and one of them (Scarlets) even has a distinct aesthetic that separates it from the other two.