I dunno what’s actually going to happen, but my prediction for the end of the expansion is:
Following the events of Shadowlands, the Shadowlands are going to decide that, with everything that’s happened, they need to be more involved/attentive to the goings on in the world of the living.
To that end, each of the Covenants is going to appoint someone to be an emissary to the world of the living. Whether that means actually bringing them back to life, or simply a dead person in the living world(like when Draka was sent on missions).
As for who is going to be sent, I think it’s going to be the Lore characters that each of the Covenant campaigns centers around. So:
Uther for the Kyrian
Kael’thas for the Venthyr
Draka for the Necrolords
Ysera for the Night Fae(or possibly Tyrande if she ends up dying before the end of SLs)
Now, their purpose is to just “keep and eye on things in the mortal world, so that when this crap starts happening again, we actually know about it BEFORE it ends up in Shadowlands.”
As for what they DO in the meantime, that’s probably up to the emissaries, and however they want to spend their time in the mortal world, as long as they are reporting back to their masters in the Covenant. And will likely be different for each character.
Best case scenario for Kael’thas? He becomes leader of the San’layn and join the Horde! Maybe Uther rejoins the Alliance. Does Draka go live move in with her daughter-in-law? Who knows! The future is wide open!