What would keep you coming back?

Hey yall. I am currently making a mystery campaign app for groups of friends. These campaigns range from 1 day- 1 week as of now, but obviously the biggest hurdle is finding a way to keep the user retention up.

So if you have any suggestions on features I could include on this app to make sure that you would want to come back to see the end of the campaign, let me know! I’d love to hear all of your awesome ideas. Thanks.