More or less in order of when I first had them:
-Fantasies that, by some magical means, I got to experience multiple special days in a girl's development, namely: first noticing swollen nipples, first noticing pubic hair, first time shaving my legs, wearing my first bra, first period, sometimes outgrowing my first bra, and losing my virginity.
-Gynecomastia fantasies: That I as a teen began developing breasts, which I hid at first with bandaging and so forth, then I was in charge of babysitting a baby whose mother wasn't lactating, and I secretly let it suck on my nipples as a pacifier, but then my breasts began lactating, which I couldn't hide for long, and then I was actually, for the baby's health, made to nurse it full-time. In the original version of it I remember a final scene of me in a knee-length white dress, with legs and armpits shaved, openly nursing a baby with my then fully womanly breasts, in front of a group of women, also in dresses. In later versions, I fantasize with first wearing a nursing bra and a woman's top for comfort and gradually switch to wearing women's stuff, or alternatively as part of my role as wetnurse I'm made to wear a maid's uniform, in either case my whole body feminizes over time (and, if not made to wear a uniform from the start, as my figure changes I start wearing panties and women's pants and one significant day I give up on any pretense of maleness, and instead of dressing in tomboyish outfits, I shave my legs and armpits, put on a summer dress and makeup and high heels) and shorty after eventually weaning the baby I have a period. I eventually get married as a virgin bride, we move to my husband's hometown, he knows I wasn't always a girl but nobody else there does and nobody has any reason to suspect as I give birth about 9 months after the wedding.
I'd love to hear others' fantasies.