Those that died in burning coming back to life, the return of lands of the night elves with the possibly Stonetalon Mountains and Azshara being included, and a guarantee the Horde will never take their lands again.
Alternative solution is the full eradication of the Horde because chances are if another faction war comes around it very likely that the night elves get attacked first or be the cause the next faction war.
Another alternative is going back in time and erasing the BFA war narrative from lore, this includes events leading to the Burning of Teldrassil. (aka World of Warcraft: Chromie and Her Homies Strike Back)
And as a Horde character who is acknowledged is how bad the Burning of Teldrassil, I sees this not an easy issue to resolve and can spell more doom for Blizzard if not resolved.