I noticed something interesting actually, it's only a few people in this forum who are obsessed with the idea of Turalyon becoming a villain. Everywhere else online or in-game, he's barely brought up ever in the first place. And I keep asking myself, towards those people: "Did you guys not play Legion, not see Turalyon swallow his pride to team up with his Goddess' killer?"
Regardless, we already have a recognizable and iconic villain for the Light faction: Yrel. She and the Lightbound have already been set up as the major villains of a future Light plot to take over the Cosmos. Either Turalyon or Calia becoming villains would only deprive Yrel of the spotlight she's entitled to.
Furthermore, Yrel makes for a much better antagonist in this kind of storyline, as it adds an element of tragic that is absent with Turalyon and Calia. Yrel's connection with the Naaru was gifted to her by Velen upon his sacrifice, she replaced him as the prophet of the Naaru; and now, because of this fact, she is overwhelmed by visions of the Naaru who are sending her on a cosmic crusade. This is why Yrel works as a villain of the Light, while Turalyon is just an asspull for a very tiny, small, little vocal majority that still wants the Alliance equivalent of SoO.