I've been thinking about a problem I had for quite a while now: Contact information that are out of date from people that I haven't talked to in a while or are missing altogether (like postal addresses) because I didn't add them.
Here is the core idea:
An app that works as a better address book but can automatically inform your friends or certain groups whenever you change your profile (job, address, phone number, website, important milestones etc.). The app will automatically push the update to their address books or send them a notification when they don't have the app installed. The same will happen, when they update their own information.
Would you use it?
Would I use a standalone app that did this? Chicken and egg problem: you've got to make your app useful to me even if none of my friends use it.
If a particular friend also uses your app, can you mark their contact details as being guaranteed-up-to-date?
If a particular friend doesn't use your app, can you remember when I entered their details and keep track of how stale they are? If I text them and the text doesn't get through, can you highlight their number as possibly not working any more?