Cool detail most probably noticed, but the golden coral pattern on Mya's face is very similar to the leaf pattern that appears on Cartethyia when she overclocks.

My theory about the current story is that Mya was taken after she won the Bloody Agon in the attempt to artificially turn her into a resonator of the Sentinel/Threnodian or just general dark tide experimentation (and I also suspect she ended up that way because she was supposed to throw in the Bloody Agon but won instead). Cartethyia traveled to Septimont probably somehow sensing that the Threnodian is trying to come back there and it's using Mya as a vessel. The beginning sequence with Carthy is still in the regular version of Septimont and the moment the dark tide spills over and touches her, she gets pulled into that other world. This is reoccurring imagery from her 2nd trailer, in the illusion when she strikes the liquifying statue and gets pulled into that dark realm and practically fights the threnodian turning into the dark tide itself, which is also what happens when she fights Arsinosa, it's all just made out of dark tide. Another interesting detail, the yet again other reddish dusty version Rover ends up in during the finales also shows up in Carty's 1st trailer, when she sees the vision of herself blowing up the town by unleashing her power.