My girlfriend suggests a menstrual calendar app that enables men to predict women's moods, especially the days they're most amenable to amorous solicitation, and generally because it's good for men to know their partners' cycles. A woman might enter the data herself and share it with selected parties, or men could try to maintain their own records on behalf of women who aren't motivated to do it. There is also a monetization potential for advertisers insofar as women are more inclined to pamper themselves at certain times.
Especially since women's vocal range is affected by their cycle. I thought to use DSP to help determine their cycle and facilitate pregnancy or help avoid it. It turns out getting the proper audio baseline for the vocal range of women is tough and they'd be saying weird words into their phone daily.