Backstory: Small, micro communities are becoming popular as well as people's desire to move to planned communities and small cities. In the Baltimore/DC area, there has been a growing trend for people moving into these "all in one" communities in the suburbs. I lived in one for a few years and it was great, I formed a lot of close friends and loved walking to local shops. Everyone in the community joined a facebook group or group me chat, there wasn't a good platform for community based communications.
The Idea: Online community/small city platform.
Deep Dive: Building a platform from scratch, you would have to start with direct messaging and open forum discussion, similar to a facebook group. Once you have a baseline feature set, you can begin to build community items such as custom events, yard sales, kid play dates, adult game nights. Starting in a community that you know will help add individual features like pinpoint locations and up to date pictures for locations that can't be matched on any other platform. Giving people that intimate/backyard feel is where a platform like this will thrive.
Validation: I've used private facebook groups and groupme chats for things similar to this.
Monetization: It would be difficult to charge at first, you could run ads. As your feature set grows, you may be able to tie into HOAs and charge a few for larger communities that a HOA would be willing to afford.