The app knows the user's location and by subscribing to the feed from https://github.com/thomersch/openstreetmap-calendar (or the calendar in the wiki), using the new notification system, the app could notify the user of local community events.

Optionally, the user should be able to completely unsubscribe from this in the settings.
Optionally, the user should be able to add the event to the calendar on his smartphone with one click.

The notification button should only be used to hint to new events, what is probably necessary is to implement some kind of calendar view which would then pop up and show all the events. This might be too much work for what its worth though, as usually, in one city, there is usually also only just one recurring event (i.e. Stammtisch).

Before this is started, it must be ascertained if openstreetmap-calendar actually provides the lat-lon of the event, or just the city name. If the latter, a geocoder mus be involved, making the whole thing more complicated to implement.