I think this is a very tricky question.
The ‘eye for an eye’ approach is tempting, but the opponent here is a playable faction, with its own fans and real people who’d be impacted. And an actual equivalent to Teldrassil (as someone said above, where their faction leader is reduced to tears over a society-shattering, mass-murdering event they are powerless to stop) is a terrible thing to inflict on a playerbase.
Of course, it’s just as easy to think ‘well, Blizz broke all those reservations to hurt us, so why shouldn’t they break them again against the Horde this time?’ - and that bitterness is hard to shake off. Especially when one of those few posters pops up arguing that night elf fleeing civilians totally deserved to die a horrible death because of something unrelated to them.
sigh
So, here’s what I think would at least get close to salving the open festering wound that is Teldrassil’s destruction:
Show the night elves retaking Darkshore and Ashenvale. Don’t tell us through Twitter after the expansion ends - craft a scenario, for the player to actually play through, showing the night elves at the height of their competence and effectiveness.
Let the night elves ambush the Horde this time, taking out camp after camp, calling the trees and the wildlife to attack their foes. Let some of the Wild Gods join in - if not in person (Though I think Cenarius at least should clear out a logging camp or two) then at least show them giving their blessings to the night elves.
Use all the fun assets from the Darkshore warfront - Keepers, Dryads, Faerie Dragons, Chimaeras. Show the unified Night Elf force that WCIII fans have always wanted to see in this game. And let Tyrande show off her new Night Warrior power in a plot-significant way.
Show them retaking the whole forest (even if the player only participates in a few battles, show the messengers from other points reporting victories to Tyrande), and show some plan at fortifying it against any future attack. Giant thorn walls or impenetrable undergrowth, swarming with angry wisps, blocking the entrances to the Barrens and Azshara or something like that.
And I would like to see some Alliance support in this campaign, especially if the writers want to reinforce the idea that the night elves don’t have unlimited troop numbers (though when have numbers ever mattered?), but I would like to see mostly worgen support, then draenei, and after them some humans/dwarves/gnomes/etc.
In other words, since so many night elf fans feel like they’ve only been used as the Horde’s punching bags or damsels in distress for the Alliance to rescue, show off the night elves being competent and dangerous in their own right.
(I’m presuming this happens during a patch where the Horde is dealing with their burgeoning civil war, so they - and the Horde player - have other internal battles and plot goals to focus on instead.)
Alternatively, if the writers are really planning on the ‘Blue EK/Red Kalimdor’ (which I hope they won’t do), then I want to see a huge, in-depth campaign for the night elves (and the Forsaken on the other side) to build a new capitol which serves as the next expansion hub on whatever continent/island we visit next.
Let this new city start as a small hub and let it grow over a few patches as players quest and contribute to it. Have lots of NPCs expounding on their cultural lore during these building quests. Use all the fancy new art assets, like the new docks at Auberdine or Val’sharah trees and buildings. Show the night elves (and the Forsaken in their mirror) moving forward and reestablishing themselves in the world.
And lastly, I would like to see the night elves involved with taking out Sylvanas (assuming she is getting at least ousted from the Horde this expansion). They don’t have to be the ones to strike the final blow, they don’t have to be the only people involved - especially considering the night elves will be involved in tackling Azshara - but I want a night elf lore character explicitly stated to have been important in the effort to take Sylvanas down.