What kind of transformation most appeals to you and how would you behave, once transformed?
There are many types of actual and physical transformations: from man to woman, shrinking or becoming a giant, becoming another creature or regression to become younger, to a bimbo or a slut, or maybe just a part of your body or physique which transforms itself. Maybe something else entirely?
Not sure if this is the kind of answer you’re looking for exactly but here goes:
I have a breeding kink (I’m a guy). For me, the most appealing transformation is a woman’s body during pregnancy. The way her belly expands, and her breasts become big and engorged, and she lactates. It’s really hot to me that a woman would open herself up to you and accept your semen knowing that her body will change and she will have to carry your offspring.
That appeals to me massively.