This is speculation but:
- Tyrande’s Night Warrior power is tied to Ardenweald
- The Winter Queen’s existence is tied to Ardenweald via the Heart of the Forest; the weaker it gets, the weaker she gets, and vice versa, and this Heart is tied to the “lock” on the Maw. (Campagin Questline)
- Winter Queen/Ardenweald will be restored by Tyrande letting go of her vengeance and releasing its power to Ardenweald
at the same time
- Excluding Tirna Achiad (Heart of the Forest), there are a total of 9 Tirna Trees (Eas, Glayn, Rowen, Vaal, Scithe, Fenn, Kaithe, Marwol, Noch)
- During questing, we learn Tirna Noch is “long dead” and they never bother to check it out; turns out the Drust have colonized it and restored the Heart of Tirna Noch with anima for their own benefit
- There’s a part of Tirna Marwol that is closed off and looks raid-esque.
- These trees are what allow for reincarnation; they gather anima to feed the Wild Seeds.
And at the same time:
- On Azeroth, World Trees can be bound to World Trees in the Dream
- There are non-World-Trees that have Emerald Dream parallels (e.g. Tal’doren vs Daralnir)
Therefore it stands to reason the Winter Queen and Ardenweald-bound Ysera can give their “blessing” to restore Teldrassil.
Also consider Undercity is currently blighted and we are being given a very Forsaken-Scourge-esque zone with a “House of Plagues”.
I thus speculate, and have been, that the end of Shadowlands will lead to most of BFA being “undone”; Undercity and Teldrassil restored.