So, as it says on the tin, this is a game about magical girls. Namely, magical girls with extra equipment. In this scenario, every girl who makes a wish becomes a Futa, in addition to gaining magical powers. Other than that, it is in essence very similar to Puella Magi Madoka Magica, the anime, save for a few key details:
Corruption: Being a magical girl, and a futa, severely ups your sex drive. You build up lust throughout the day and must find an outlet to satisfy yourself. Masturbation only helps to keep the edge off, to fully 'cleanse' yourself, you need to have an encounter with someone else. Those who don't find an outlet, or who fall prey to a witch or its familiars, become a witch themselves.
Witches: Witches, and their familiars, mostly prey upon the lusts of their victims. Magical girls can expect to be assaulted both physically and sexually during their fights.
Combat: The system will use D&D 3.5 as a base framework. I'm thinking everyone will start at level 6, with about 10k in starting gold to spend. Things can be modernized to an extent, if you want to use the stats of a crossbow for a gun, by all means go ahead. A few changes will be implemented, however.
HP: Rather than representing health, this is how close your character is to orgasm. If a witch or its familiar causes your character to orgasm you take a negative level, are stunned for one round, and then may resume fighting. Anyone who dies to negative levels becomes a witch.
That's all I can think of for the moment, really. It's sort of a rough draft at the moment, a work in progress really. If you have any questions feel free to ask, it'll probably help me flesh things out a little bit.