It is incredibly tempting for me to just say “it can’t be done”. And in truth it really can’t. Not with the way the story has played out up until this point. The reason the Horde could recover from the WC2 era was because there was a clean break between them and because most of the modern Horde had nothing to do with the Second War.
But now it is the current Horde that did the Bad Thing. Teldrassil is a looming cloud that will forever be cast over the faction. It cannot be undone or forgotten. It cannot truly be redeemed.
This is made worse by the fact the Horde hasn’t done anything useful since… Wrath? I think Wrath was the last time the Horde got to meaningfully use their military to help defeat a major villain instead of smashing the Alliance.
So my proposition; Retcons. Lots and lots of retcons.
Not removing BfA from canon, though I’d certainly love that to be the case. We need examples of the Horde doing good things in previous expansions. Particularly Legion. Maybe write a book about the Tauren and orcs waging war against the demons while the events on Argus went on. Maybe showcase the Horde fighting against N’Zoth in a bigger way during BfA.
Basically just go back and inject some Horde heroism into places where the Horde logically should have been involved but just wasn’t on-screen.
Then go back and flesh out the Horde’s reaction to Teldrassil. Show Lor’themar, Baine, Thalyssra, Rokhan, etc. etc. as wanting to stand against Sylvanas but not being able to. Maybe show them going against Sylvanas’ orders on the down low to quietly subvert her rule. Anything that makes them look less like gullible bafoons and more like heroes with agency of their own. And most importantly disprove the notion than the majority of the Horde was pro-Sylvanas.
Basically just… Go back and write the Horde how it always should’ve been written.
Once an actual foundation of Horde heroism has been set we can start building back up from there. Maybe have a big moment in Shadowlands where the Alliance is being overrun and the Horde charge in to save them.
Teldrassil will always be a scar, but at least then there’d be enough good in having the Horde around to make them a defensible faction as they’d be instrumental in keeping Azeroth safe.