Illidan as a character still has plenty of legs and very little closure. We didn’t see him on screen with Malfurion or Tyrande at all. He’s yet to be reunited with Azshara. Then there’s the entire business with his destiny being THE champion of Light. AU X’era is still floating around, and I imagine she will want to claim him at some point. Add in the fact that he seems to be unofficially Azeroth’s defacto champion.
You forget that BFA had these special mission table quests that was part of the bigger War Campaign that was happening behind the player's backs.
Stuff including the Alliance retaking Southshore, Horde occupying Strahnbrad as a vantage point, The Bloodfang Pack taking control of Fenris Isle... All this stuff is also canon and actually "progressed" the world... like after the war ended, The Alliance is now cleaning up and rebuilding Southshore again from being a plague/slime-infested wasteland.
Because Merithra is the new Green Dragonflight Aspect, and Ysera isn’t. Why would they completely invalidate their new Aspect like that? That isn’t how to build a character. It essentially tells us that she doesn’t matter.
Ysera is likely going to play a role in the future expansion in regards to the green dragon flying to Avaloren and starting a new flight there.
Ysera is definitely tied to 11.0 as she has zero purpose so far in DF and they wouldn't rewrite her entire story over some twitter butthurt (unlike what a lot of people believe, regarding Merithra being aspect, many people think it was a hard pivot based on the reactions to datamining... while Merithra being aspect was set up in BFA)
Could be Un'goro on account of it being a crater. Could be in Scholazar on account of it being a crater, having water in the middle, and being personally overseen by Freya.
It will be wherever the writers say it is, and will just pretend like the other options were just coincidentally similar.
The world revamp dream will never die!