I'm an introverted guy and have a lot of hobbies. I love to be alone and do my things. Therefore I often forget to keep in contact with friends and acquantainces. Months later the person comes to my mind and it feels odd to reach out now, months later.
- I make a list of all the people I want to stay in contact with and when I had contacted them the last time.
- The app shows a sorted list with the last time I had contact to the person.
- The sorted list is the core feature.
- I could add a notification, e.g. "You didn't reach out to Alice in the last 30 days.".
- I could add a custom notification time, e.g. default 30 days, grandma 7 days.
- I could add a questionnaire, e.g. "Whom did you contact today?"
- Because reaching out to friends is mostly done on smartphones, this should be a mobile-first app.
There are a lot of reminder/calendar apps available. I've never found one that emphasizes the question "when was the last time I did something".
Freemium: max amount of people to add, custom notifications etc.