Tracking periods is very important to many women. It would be very nice if there was an option to turn on "period tracker mode."
A simple check box every day that we click if we are having a period and then a prediction of when our next period would be based on how long we set our cycle to be. If the calendar could adjust our cycle length based on when we track, that would make it as good as most period trackers out there and eliminate the need for a second app. That would also be a good selling point. It's annoying to need a whole different app for this one thing!