I think this is all a setup to kill Thrall as impactfully as possible

Blizzard traditionally has decent writing from a birds-eye outline level that falls apart in specific moments of execution. I think this is what the war of thorns is, and I think the chess pieces are in place to predict the next few moves. Sylvanas is indeed looking like Garrosh 2.0. So much so, that in the next patch or so, it will draw Thrall out of the woodwork to either help depose her or at least meet with other horde leaders to resolve the situation. In fact, it is one of the only scenarios where he would be willing to come back and would be a major character in the plot again. At that point, only after Kul Tirans are officially part of the alliance (so a full patch from now at least which is why the pacing feels so awkward now), Thrall will be murdered publicly by Jaina or a Kul Tiran NPC in retaliation to the WC3 stuff. That will reignite the war between factions even more, and the horde will have its own tree burning moment of outrage.

Sylvanas won't become the primary antagonist of the expansion. My guess is that she will be deposed as warchief but not race leader pretty early on, and will play a part in the old god story to come (possibly as a cheesy savior character by the end). This warmonger Sylvanas merely is a patch long rehash of the Garrosh story so it can then spin off in a different direction.