I just answered vampire, but yay let me answer the rest!
Witch: Oh man, I don’t know!! I want to change so many things. Maybe get rid of cancer? No more world hunger? Stop pollution? Expand space travel? Anything that adds to the happiness of living things sounds good.
Siren: I’d probably make my sister(s) choose healthier life choices, tbh.
Vampire: Hmmm….that’s a really tricky question. I think that I might choose immortality? I think it would be pretty lonely, but interesting to watch the world go by. I would only choose it after I did some really extensive magical research into the afterlife. I don’t want to miss out on the next level!
Mermaid: I would go pretty darn far to keep someone I love! I would like to say I wouldn’t hurt others, but I also know how hard it is to lose someone and how crazy that can make you. I would say I would draw a line after using dark arts or sacrificing my self in some way but before destroying the world?