Unfortunately, just based on "what is the most obvious thing that Blizzard is telegraphing" the "world tree" is probably Avaloren.
Why it's invisible IDK (2x invisible force field? it's Atlantis?) but I think that Avaloren will appear in Dragonflight and not in the future.
I'm also a strict advocate against the Night Elf tree being on another continent "cus da Greens like them" as it sets dangerous precedent for how disconnected the races of Azeroth might be in the future/pretty much destroys hopes for an EK/Kalimdor revamp (northrend/pandaria is never happening imo).