Might be neat if evil vampire Rey had the Sith eyes.
Edit: Something like this mockup; or maybe subtler. Or even more vampiric.
Then again, making vision Rey even more obviously evil just sort of reinforces the general idea that Rey isn’t actually tempted towards the dark side at all. It’d have been more compelling, in my opinion, if there was something in that “cave” that actually appealed to her in some way. Alas.
EBSynth test. Unfortunately it squishes the pupils around so I’d have to do those manually, I think. Still, plausible if desired.
Hmm, personally, I think this might be a little much. BUT! I think it would work if her eyes only change for that brief moment when she goes all Bilbo/Gollum/shark on herself. Kind of like how Anakin’s eyes only turn when he’s being super-duper evil, but are normal the rest of the time (e.g. when he’s
choking chatting with Padmé). I agree with your second-thought that it strips the nuance from her temptation; she’s just very obviously an evil monster now.
Plus, if it’s only a brief flash when she “submits” to the dagger, and only a brief flash later on, that would reinforce the two points, and help to ground them a little more, I think. Adds a bit of weight, like, “Oh sh-t, she’s actually falling to the Dark Side!” (Rather than, “I guess she’s already fallen.”)