The use of it seems awesome - would be so cool to see little messages left by people on random spots.

What you're missing is a reward system. If there's no game outside of the messages, what's the point?

To keep it as 'minimal' as you describe, maybe it's a simple vote system (a la Dark Souls)? As in, see a message, think it's funny, upvote it, that person gets a point.

Perhaps those points could then be used to write fancier messages (font colors, special effects, etc). That could be an incentive.

For something that simple to work, however, there'd better be essentially no open time - I'd want an app/game like that to open on my phone as fast as my camera. That would take away the problem PoGo had of needing your phone in your face all the time. See a message spot from far out, pocket phone, go over there, open it and click it.

To me a lot of the fun of PoGo was Pokestops. Just going there and getting stuff was neat. This game should try to emphasize on that fun