It does sound like an easy way to achieve the same effect as Argus had after ToS.
Personally i would have assumed a similar skybox effect with maybe some swirls or eclipses or something. But vague darkness does seem like something that is feasible.
I am pretty sure there will be something though. Not completely 100% on what. But after Argus got that good reaction after ToS i think Blizz will likely continue trying ot achieve the same with mid-expansion raids going forward.