She was just featured prominently, give her a rest. Her character became one of the most hated in lore due to overexposure and a convoluted storyline. As the old saying, "how can we miss you if you don't go away?" Midnight seems to be a part of the "saga" people seem to see fitting for her to return. Anything in the foreseeable future seems too soon for Sylvanas, even the World Soul Saga.
A better question right now would be, when will we see Illidan and Sargeras again? Illidan due to his popularity, and his return story was actually done well, and Sargeras because he has beef with the Void and a stake in the outcome of that story.
If Midnight needs some Windrunner representation, there's enough with Alleria, and let Vereesa get some spotlight too. Sylvanas' story should/needs to be over after the Shadowlands car wreck. Undeath was her curse, it became her vehicle to terrible acts, and eventually her burden. It's fitting she stays in the Maw doing whatever the hell she's doing for all eternity. Let her become a memory.