I don't know what "their traditional gender" means, but you can choose the Pokemon's gender when you start a new game! You can also customize it with items you purchase in the shop.
https://gitgud.io/argonvile/monster-mind/
Breasts are an interesting idea, you'd probably want to have new animations for grabbing and fondling their breasts, new clothing options like tube tops to show off their breasts and have rules for which pokemon like having their breasts fondled different ways! You'd probably want to also have the cosmetics be toggleable for people who like bigger breasts or no breasts
It sounds like a lot of work, but if you want to take it on yourself the game is open source. Good luck!