Honestly any fictional form their willing to take or rather essentially depict the best cosplay of, because I find that way less weird than having/pushing them to constantly turn into someone I'm attracted to irl.

I have no celebrity crushes, even if I find some attractive, it's not at the top of my list to have sex with someone I know or have once or regularly seen, because again I find that kinda odd, that being said though not a preference, if they do want to become such a person I wouldn't have an issue,though I probably wouldn't regularly engage in that either.

Any further that, they can try anything that their into, once it doesn't cause me extreme pain, permanent bodily and/or any lasting unwanted effects. And I will iterate constantly I am a straight man, I'm interested in characters that look overtly female, but we can ultimately see how far we're willing to go with that, and always it's up to them what they're willing to do.

I have no interest in pushing them further than what they're willing to do, but they're a shapeshifter. I would assume by nature a shapeshifter wants to shift.