I recently saw and loved "The Quintessential Quintuplets" and had a few stories in mind.
The first story would follow Miku between the time of the end of the anime until she opens the café with Nino. During this time, she will be honing her social skills which will teach her how to better interact with people, learn business, and enjoy herself. She will find work in a maid café, following her own suggestion, and will become quite a fixture there sharing history with patrons. Her family's parallel adventures could become part of it. Working title: "Miku's Mesmerizing Moments."
This story is my favorite idea
Yotsuba's career path is never clarified, but I did have an idea based on what little she said. She said she's an "adrenaline junkie", also is an athlete, and loves helping others. I envision her being kind of a daredevil. Whether a mountain-course racecar driver, base jumping, scaling or a mountain climber I imagine her having a following online and sponsorship to fund her adventures. Story would also follow her husband's progress from tutoring, to teaching, higher level-teaching, possibility university-level, and such. Maybe he will open a tutor-franchise. Yotsuba may become a host on some action Youtube channel or maybe a podcast. Perhaps in the end write a book and be a motivational speaker giving talks on challenges she faced along the way. Working title: "Yotsuba's Yearnings."
There is a third too: Itsuki continues down the path of teaching language and using her earlier gig as "May" it will be a framing device to have her life. Her teaching helps her keep and expand her language as "May" the food critic. She may eventually become a food critic to the point of being a culinary author for some magazines or publications of such. Who knows? Maybe eventually an industry insider or consultant. Perhaps the final act will be her designing a cookbook with the aid of her sisters Nino & Miku. Working title: "Ituski's Innovations."