There is something I would like to see in a story about a stretchy girl. It's a fetish I have only seen once in this doujinshi.
https://exhentai.org/g/1035649/648f21a076/
The bartholin and skene glands are more likely candidates for insertions than the milk ducts (at least they are visible), and a septuple penetration like the one depicted in that doujinshi is hot as hell. Might be worth a try.