Mastodon, you're talking about Mastodon, lol.

Anyway I'd build an service that provides free wifi at local businesses, then allows users to opt-in to a service that matches clients of those businesses with each-other based on common interests. Trying to encourage the establishment of hangouts / Third Places as a normal in society again.

Also possibly an application that tracks user input and the value of their input to the accomplishment of stated goals for the purpose of profit sharing. Essentially an automated accountability layer for cooperative projects and business ventures, integrated with automated project management based on Natural language processing.