Very easily honestly.
Using Midnight as the potential point because it does involve Quel’thalas, something is going on in there and we need the aid of a previous Ranger General, and we can’t use Alleria because she’s all voidy and that just makes things worse.
So someone from the Horde Council, likely Voss or Calia, reminds everyone there is still one more who is technically alive, and the Player is tasked to return to the Shadowlands and get permission from the Arbiter and the powers at be in the Shadowlands to go into the Maw and get Sylvanas for this job. We promise to keep her on a short leash and send her back when her task is done.
We go into the Maw and find Sylvanas who is refusing to leave because she found Nathanos and won’t leave him there a moment longer. To get her to come with us, we help her free Nathanos soul and get him to Oribos where he decides he’ll keep fishing souls out the Maw while Sylvanas is in Azeroth doing Midnight stuff.
After Midnight stuff is done and Quel’thalas is no longer in any immediate danger, Sylvanas goes back to the Shadowlands. She may get a moment with the Horde leaders for an apology or something, and maybe even a message sent to Tyrande if she’s feeling extra repentant, but she returns to the Maw to continue fishing souls out.
Basically we’re given a reason to bring her along, she does some things, then goes away again.
I don’t like the idea of her finishing in any short amount of time, nor do I like the idea of her leaving even for a little bit when she has a universe of souls to find, thus why Nathanos gets found and starts aiding the task (since he’s also responsible).
I acknowledge this is very similar to what was just done with Malfurion in Dragonflight but it’s really the only way to get her in, have her do stuff, then back out, while also acknowledging that she done the bad.
Feel free to add optional dialogue where you can spit on her and stuff, if you must give players a “eff you Sylvanas” option before we mindlessly do what we’re told for 5 gold.