Sooooo... I have some thoughts and questions about the twin-tailed comet. Hopefully my fellow Warseers can chime in on.
Though the icon of the twin-tailed comet has long been used by the Empire, it seems to me that the forces of Chaos have as much - if not more - reason to use the twin-tailed comet as one of their symbols. Evidence seems to show that the comet's appearance signals a strengthening of Chaos, perhaps even causing it. It seems that the appearances of Sigmar/Sotek/Valten/etc. are a reaction to the waxing of Chaos.
Additionally, the twin-tailed comet is a divergence from the norm - something unnatural.
Is it possible that any among of the forces of Chaos might use the comet as their symbol or a lucky icon? I mean, beyond the usual, "Well duh, it's Chaos, they can do anything." The Norse and others that have the most contact with the Empire probably would not, as they would still see it as a symbol of Sigmar and/or the Empire due to cultural exchange. But what about the others like the Hung or more distant Kurgan tribes? Or even disillusioned Sigmarites who have "seen the truth" about the comet and defected to the Chaos Gods?
On a related note, what about worshipers of the Gods of Law? One way or another, the twin-tailed comet signals change. It seems the symbol would be anathema to them, even as a representation of Sigmar. Yes? No?