Backstory: There are a lot of really great private and public slack channels but there's no central location to find one that fits your startup need.
The Idea: A directory of slack channels.
Deep Dive: You would build a very simple site where people can submit their slack channels. You would provide a way to sort and filter by industry and other attributes.
Validation: This is a small niche that can help a lot of small entrepreneurs find solid contacts to grow their business.
Monetization: You can sell premium ad spots or work a deal with paid slack channels to get a commission for every new member.