Re being public / private - I would default to public, but some sort of private posting or messaging add on would be useful.

There is a lot of local information that can be shared publicly and usefully by friends and neighbours etc - is the road to town flooded, when will the local doctors surgery be doing vaccinations etc. As well as marketplace type functions... All could usefully be put on a blockchain.

I have atypical in how I found people on-chain with similar interests as I had the advantage of doing the shows on MSP and running things like the Homesteaders Lists. Discord and Slack were the other way I found people with similar interests.