well first i would say any placement can go one of two ways depending on the level of consciousness the person has. the placement shows the innate potential/possibilities but it takes nature, nurture and karma to bring out the Light or dark side of them (in my opinion).
scorpio placements (moon in scorpio, venus in scorpio) or tough pluto aspects to feminine planets (moon conjunct pluto, venus square pluto, etc...) could be femme fatal or could be deeply transformative and empowering- it all depends on how you work with the shadow energy activated by the placement or aspect. also i would definitely look at the 3 liliths (black moon lilith, dark moon lilith and asteroid lilith) and fixed star algol. dark moon especially as it is exclusively about the shadow of the dark feminine.
i don't think neptune or venus/neptune produces femme fatal. they produce more of a deer in headlights, ungrounded romantic, victim/savior thing. it's pluto or lilith that adds the darker undertones. someone with only neptune may have an alluring quality but it is usually completely unconscious on their part- and their draw to darker energies that take advantage of them or play on their neptunian idealism/denial happens usually as a result of their complete disconnect from their own shadow.
just my two cents