Alternate Title: The Warbringers shorts are literally about those who brought this war to us.
We know Azshara is a villain, and after the last few events, it definitely seems that Sylvanas is a major antagonist of this plot for both factions. That just leaves Jaina.
This idea came to me when I read about people interpreting Jaina's reaction in the throne room to Anduin talking about Teldrassil as one of guilt. At first, I didn't think much of it. The cutscenes have some brilliantly subtle facial animation nowadays, but sometimes that also means that characters just aren't static if they're in the shot, nothing more. However, I went through the rest of the cutscene and noticed something. Despite Jaina having the most connection to the Arthas cinematic from Warcraft III that it references, Jaina only gets focus, and a very brief focus at that, at the very end when she teleports everyone out. This is also the only time that she has any dialogue in any of the Lordaeron cutscenes. Also, when the four Alliance characters reach the center of the throne room, she is standing out of formation and further behind them. None of this has any innate meaning, but it kinda gives me the feeling that they want her to go under the radar right now while still appearing in key moments, but it would also fit if she really did have some guilt about Teldrassil.
Another thing that stood out to me was how Sylvanas broke the Alliance's siege fairly easily and fairly early on. Her final trap and big speech to Anduin never would have mattered if Jaina had not arrived. Of course, Sylvanas could have always hoped the Alliance wouldn't have an answer to that push, but smart enough to have a back-up plan anyway, but I asked myself what it would mean if Sylvanas had planned for Jaina's arrival, because they had planned it together.
Why would Jaina work with Sylvanas?
I'll admit, this relies on another theory, but one I feel isn't really a leap at this point, and that's about Sylvanas having ulterior motives all along, and isn't really working for the Horde. Aside from her actions themselves and disagreeing with Saurfang and Baine, evidence for this includes the Three Sisters comic, where the Void is convinced that Sylvanas is working for "the true enemy", Varimathras' and Gul'dan's dialogue in Legion, where they mock the Horde about Sylvanas being just as much of a threat to the Horde, and the Chronicle, which establishes that the Void relies heavily on the Alliance and Horde staying at war with each other.
Now, the first and last of those contradict (since one presumes that Sylvanas is working for the Void and the other presumes that she's their enemy too), but my point is less about who Sylvanas is actually working for, and what that force wants. I think it's fairly safe to assume at this point that whoever it is wants the Alliance and Horde to be at war.
So back to the question, why would Jaina work with Sylvanas?
Because Sylvanas proposed a situation in which Jaina could get her revenge against the Horde without the Alliance seeing her as a bad guy for it. I doubt Sylvanas told her the whole plan or why she's doing it, in fact, she may have even gave the same sort of justifications that she has to the Horde. In Jaina's current mindset, Sylvanas saying that the two factions can never truly have peace unless one destroys the other is something Jaina could easily latch onto.
Thus, Sylvanas and Jaina both get what they want. Sylvanas gets the Alliance and Horde to war with each other, and Jaina gets to fight the Horde and be hailed as a hero (or so she thinks).
As a follow-up theory to this, I think both of them (indirectly or otherwise) could be working for Azshara, tying all three Warbringers together.
Secondary Theory
SPOILER WARNING for Battle for Azeroth questing.
We know that Azshara has her tentacles in both Zandalar and Kul Tiras. This right away sets her up as a much bigger villain (regardless of what may or may not happen to her in an early raid) than what you'd expect for someone who's had essentially no presence in the marketing outside of the Warbringers. During the questing, the Horde and Alliance overthrow her presence in their respective lands, but if Sylvanas and Jaina (the latter of which specifically becomes the new leader) have ulterior motives, this actually may have been part of the plan all along.
Picture this, Azshara is pulling a Kansas City Shuffle on the Alliance and Horde. She creates moles in strategic areas that she knows the Alliance and Horde will want to gain control of, but makes them relatively obvious, so when the Alliance and Horde take down the traitor and take over the new area, they don't expect that the real mole was within their ranks the whole time.
This could even explain why Zandalari Trolls and Kul Tiran humans are unlocked later in the expansion, because our initial victory in their lands isn't actually the end of it, and we end up having to do a second campaign to actually free them.
SPOILERS OVER
I think some of the dialogue we already have could support this as well. For instance, Jaina's dialogue in the original features trailer isn't very heroic, but it was easier to dismiss when the early marketing seemed to paint the war as a necessary thing for both factions that we should be rooting for. Now that the war has actually started, it seems almost definite that it's another misdirection by a greater villain, with both the Alliance and Saurfang being against it. In other words, this makes Jaina's dialogue in line with the villains in hindsight. Then there's also her "beware of me" at the end of her Warbringer being pretty ominous. In a vacuum, it's easily a cool moment for her, but everything else makes it more questionable.
Now, I'll admit, I think this would be a disservice to Jaina's character as well. She'd go from a peace-aiming intellectual in Warcraft III, to a guilt-motivated warmonger in Mists of Pandaria, to a straight up hypocrite who lets people she cares about die to make up for letting her father die, but I don't think it would be out of the realm of possibility. It would serve fairly easily as the sort of twist that everyone has come to expect in plots like this. First, the constant clarification that she's not a dreadlord due to the meme would help distract from her still being a secret traitor, and most people's eyes are on Greymane as the one who would betray the Alliance for the sake of revenge.
To be fair, her being an antagonist may not be the end of it. I'm vaguely aware of her having an arc in Kul Tiras about confronting her demons, so it's possible she might have helped start the war, but ultimately come to regret it, but that's another theory for another time.