In the episode "ChaotiKings", Kaz was working his way through the puzzles of Castle Pillar. The final puzzle to get the Mugician locked away was to pick the color that didn't fit with the rest. All of those were Tribe colors he recognized except one: purple.
This leads me to believe that purple is or was the Mugicians' Tribe color. But Sloan's theory still gives me even more questions. What if the Mugicians haunting Stone Pillar and Rao'Pa Sahkk could be resurrected? Were they the ones that built Kaizeph and Dranakis Threshold? What would they look like? What could they do?
And going back to the tributes paid at the Eye of the Maelstrom, what if they're not quite as extinct as everyone thinks?