I've consolidated my story idea drafts from rpol.net, and put them here for people to considered.
Numbers marked with stars are cravings!
*1.) My character is part of the stage crew for the local high school. After a minor fire in the costume room, my character goes out antiquing for a new standing wardrobe to store some of the womens' costumes. My partner would be a part of the school acting troupe and notices that when she puts on a costume that was hung in the wardrobe, she becomes the character to a degree! Not believing her I put on the self-same costume, and shock, she was right! Now, what could a couple of board, imaginative teens do with something like this?
1.1) As a variation: We inherit from our grandfather a large steamer trunk bigger on the inside than the outside stuffed with costumes. Why our grandfather left us the trunk is part of the mystery.
*2.) My character is a transfer from a big private school. Sadly the new school doesn't have the guys version of the sport he plays (Typically, Cheering, Volleyball, Gymnastics, Softball, and Synchro. Swim at the high school level.) But the Coach/Team Captain sees my drive and inquires at the school district office. Turns out, this isn't the first time this has happened, and a remedy is available. By use of a magic spell, my character can become female enough to compete w/o any undue advantages!
*3.) The school is an old school, what the Japanese call a tsukumogami, that has developed a soul and intelligence of its own. This school tsukumogami, is fond of her students. In fact so fond, that she selects a few of them and asks them to become teachers at her school every year or two. It's not skill that's important, though possessing skill is helpful, the school can gift students-turned teachers with skill. What the school looks for is students that already have the knack of the art of teaching, of interconnecting with people. My character is asked by the school via the principal to become the next girls' gym teacher, because she thinks he's mature enough for the job. Students-turned-teachers, get salary and benefits, a car, and a housing and clothing stipend. The transformation occurs whenever the students-turned-teachers is on school grounds, or doing something adult (like driving, or using their credit card as examples.)
*4.) My character becomes a magical girl or pink super-sentai, because when approached by the magical mascot, my girlfriend who the mascot was talking to, passed out, and that monster of the day isn't going away anytime soon! What is the true measure of a hero? What is the price of power?
4.1) As a variation: a Kamen Rider/Masked Rider - Pretty Cure crossover. Can a girl be a Rider? Could a guy be a Cure?
5.) My character is suffering depression, and as a token of kindness my little sister gives her big brother her favorite book, calling it a self-help choose-your-own-adventure book. After reading a couple pages, my character is whisked away into a world of fantasy. A girl's fantasy at that! My sister is there too, as an assistant/guide, as that's how the user of the book pays off their debt for what the book wrought them, by choosing and guiding the next to receive the book.
5.1) As a variation: while the story world might be a fantastical story made for girls, the setting is science-fantasy - like Lyrical Nanoha.
6.) Due to severe space weather, A group of high-school/college students attend school in a large spaceship, and they are the cream of the crop. A crisis arises, and the she school ship is caught mid-hyperjump in severe space weather. Now stranded in lifeboats without a hyperjump gate, floating on the edges of the starmap, a pre-recorded message informs the students that their situation was a prepared contingency, as there were too many 2nd and 3rd children of hypercorp board members in the core space territories, and this typically lead to problems of inheritance. Standard procedure is to exile the excess 2nd and 3rd children, and to encourage the youth to devote the rest of their lives to expanding the reach of the human sphere of influence so that when the rest of humanity catches up to them in the Wild Fringes of the galaxy, there might be starmaps made and perhaps colonies/habitats established so the hypercorp execs wouldn't have to do the hard work of colonization and transforming.
To make matter worse, the Crisis happened on the day that she school-ship was doing annual health exams. Since it's necessary to get undresses for oh... 5-10 minutes or so in the course of the exam, guys report in the morning and girls report in the afternoon for privacy's sake. So when the Students scrambled for lifeboats, they were largely segregated by gender. Worse, beyond the heartache of girlfriends left behind because the lifeboat's Master Computer sufferers a Zeroth-Law (of Robotics) conflict.
The First Law of Robotics states that by action or inaction robots/computers may not allow harm to humans. Because there is now a large population of only guys on the life boat, the Computer realizes that the population of the life boat... say 50-200 boys depending on your preferences... would die out in 80-100 years and that this was a 'harm' condition. The hypothetical Zeroth Law of Robotics says that it's okay to harm some humans if humanity as a whole is bettered by that harm. (Example: a computer might conclude that it's okay to kill Hitler to end WWII, despite the 1st Law's prohibition against murder.) The Computer concludes that to alleviate the 'harm' condition there needs to be girls, enough to allow for a diverse reproducing population. The Computer notifies the student body, and apologizes asking for volunteers as it doesn't what to 'excessively harm' the people it serves, but it's programming requires it if nothing is done.
(<whistle> Wow... that was long! Didn't seem so when it was an idea in my head.)
*7. Alice in wonderland is a mantle, something that is given from person to person as the years go by. After receiving the mantle of Alice, one can walk/crawl into any mirror big enough to fit you and you'll be transported to Alice's Cottage. There, one can dress in Alice's clothes and become Alice. But if you stay in wonderland, the wonderlanders will figure out that you're a fake and rip the clothes right off of you, turning the victim into a Blank - a sort-of zombie that lack identity and desperately want to be -something-.
*8. A holiday was long forgotten, and on the hour of the forgotten holiday the the neglected god, a cat god, curses the school that was built over his shrine. Each day a girl will gain cat ears and tails. The girls will be chosen at random unless a volunteer steps forward. The Cat God won't refuse multiple volunteers. Each subsequent day at sunrise and sunset, they will gain another cat feature, until they are sufficiently feline (about 70-80% feline) for the Cat God to marry. (Bad End!) My character being noble, and a little foolish, volunteers to be one of these girls, by cross-dressing. The cat god happens to amused by this, and answers the deception of my cross-dressing by making it a truth.
9. A rich successful old man full of regrets now rues the mistakes he made in his life, when caring for his mid-stage Alzheimer's Syndrome wife, she gives him a magic amulet/belt buckle, saying she forgives him, before returning back to her normal semi-lucid state. When employed, the device gives him a 2nd chance, by being a superhero. Scratch that, a super-heroine. A teenage super-heroine. Apparently, he was worthy of a 2nd chance, but to make sure he won't make the same mistakes over again, his gender is reversed in his young form. Now with a year to make his choice to due an old man or live as a young girl, the world in new and unfamiliar, but exciting.