What do you think would work well? What do you think the pitfalls might be?
Not cynically. Just, is this a hobby for someone? If so, awesome. In my town we have a WhatsApp org for local musicians that helps with gear exchange, gig promotion, collaboration, etc.
Restaurants and bars and event spaces use a myriad of promotion vehicles. Other local businesses. The paper. Official announcements from the town. Other events. Etc. Same with the groups playing.
The ways to find this stuff aren’t amazing but not so bad that I’m searching for another app on my phone.
My point was that even as an avid music fan, I don’t hear a lot of folks looking for another app for this but maybe I am way underestimating what’s possible.
The bars in my city don't have public APIs. If you start small and local, you could maybe get buy in from some places, "Send this address an email with event details and I will help you get butts in the door!"
Also if you build any app that lets you meet new people, especially new people with compatible interests, Match Group will buy you out and turn whatever you built into another tinder.