I don’t really care about the vault one way or another (isn’t it supposed to just be bad luck protection for raiding/mythic+?), but I would like to see a Torghast style system return as a permanent game feature instead of an expansion-specific feature.
Something more generic like a Caverns of Time or Kirin Tor magic themed area that new themed zones can be added to every expansion.
Don’t tie it to primary gear progression. Just let me earn a currency to get mounts and cosmetics and maybe the occasional gear upgrade or profession thing.
Torghast was a fun type of content (especially as group content that didn’t require a tank and healer every time). Making it required for legendary progression is what killed it.