Thank you for the feedback on what is most evocative! And thank you for the very practical feedback on density.
Density is definitely a problem. This ideally wants to be a social network, but gaining meaningful adoption for a social network is difficult. Of course every user is *also* their own fellow user within the same locality and so could at least talk with their own past self (if they so desire, haha...), and also all global users also constitute another locality, though not as immediate of one. I think even seven users on the global scale could make for an interesting experience since it would basically be a message room except you see from where messages are being sent. Users could set the radius for the messages that they want to see. As soon as there are enough users within a certain radius, one could start looking at the conversation happening more immediately around themselves. For the fully realized idea of sitting down at a park bench and being able to read the messages that someone left there exactly there last week... this would be quite far off even with a relatively successful implementation, and maybe only possible in places like New York City where there are many famous everyday landmarks. A more realistic middle ground is seeing what people within a few tens of kilometers of you have been thinking about, since even though this is not a trivial distance to travel it still is traveled often enough that different places in one's everyday life (like work vs. home) would expose one to a different pool of voices.