I really like how evocative this part is "Imagine Pokemon Go, except you go around down to see what thoughts you can find. A chatlog for life which you can use to leave notes anywhere in the world, but specifically for others in the same time and space." - definitely the best part of your pitch.
I do wonder if you'll have a density problem here - what's your expected user scale, and the expected concentration of users, to make this a fun experience? but this seems solveable if you focus launch on particular cities first, potentially.
But definitely a fun project to think about and I can see it creating something valuable for people!