As another thought: I think the idea of revenge for Teldrassil is difficult because of different overall priorities and mindsets between the factions.
I can’t speak for anyone else, but I tend to lean Alliance because, especially for players, they are largely the defensive faction. While questing for both factions is primarily to defend already-settled lands, the Alliance has more zones where their land is considered ancestral; where they lived relatively peacefully for a long time, and have deep roots and connections with the territory.
The story for the Horde races is rather different: some Forsaken show deep ties to Lordaeron soil and their history with it, but others don’t. The tauren were nomadic, the trolls lost their island and haven’t been written with a strong drive to retake it (or even mention it much), and the orcs are forging their place in a new world. While they’re not devoid of a personal connection to their territories, more focus is put on expansion rather than reclamation. (The blood elves are a big exception, but past their introduction, the lore hole has kept there from being much worry about losing Quel’thalas.)
And, of course, the initial zone imbalance made this divide worse, since balancing it made the Horde more expansionist and the Alliance suffer defeats (or just get defeated and never hear about it). While this rebalancing was sorely needed, for many Alliance players, this made them even pricklier about losing more territory, especially to the Horde - because now that things were balanced, there definitely wasn’t going to be any good way for the Alliance to get what they see as recompense.
So this sets up a situation where the Horde has a tiny amount of locations with as much impact to the average Horde player as places like Teldrassil have to the average Alliance player.
And as a side note, because this is even more unscientific than my other conclusions: I think that, based on the not-really-representative forum posters and people in chat that I’ve seen, more Horde players than Alliance players seem to prefer sounding tough - so even if something happens to them that’s as emotionally cutting as Teldrassil, they’d probably show less of a reaction, leading many Alliance players to think that an equal blow hasn’t been struck.
So, in conclusion, I don’t think anything can be done to really ‘equal out’ the Burning of Teldrassil.
Anything equal numbers-wise won’t effect the same reaction the Alliance had, and anything equal in emotional response is probably impossible, or would be horribly imbalanced.
In raw numbers, the Alliance has sacked one Horde city (though without punching through an equal amount of territory and civilian towns to get there, while looking stupid against the blight, and having Sylvanas still make the final move) and successfully struck a second city and killed Rastakhan. They’re told that they’re winning on all fronts and close to victory. But it doesn’t have the same emotional punch.
In terms of emotion, what valued things does the Horde have left? Orgrimmar? It’s been sacked once - twice would be an insult, but it wouldn’t be shattering a pillar of the Horde’s identity. Kill the warchief? They’ve been playing warchief roulette already - depriving them of another wouldn’t be impactful. At this point, anything removed from the Horde would reduce them to the state where they need to seize something to build back up, which negates the revenge aspect and leaves the Alliance howling for more that the Horde doesn’t have. Wag their finger and tell the Horde not to do it again, for realsies? We did that already, and it’s already been trampled over and forgotten.
For a largely defensive-minded group like the Alliance, what makes up for losing -again- on the defense? For having so many civilians die on your watch, with a quest made to reinforce how hopeless is to save them all?
I don’t think any offensive action can make up for that. And the players have retaken and fortified their lands several times only for the next plot to come along and wreak havoc again, so it’ll require a lot of hard work and good writing to make any defensive action feel like it means anything more than creating a shiny for the Horde to knock over the next time they go a-rampaging.
Sorry for the ridiculously long rant. Just… ugh. This plot was a terrible idea, and there’s no way to wrap it up without insulting one or both factions.