Things it can do / Characteristics

people can leave messages that are associated with location that other people can find when they enter the same location

types of messages

person to person
person to persons
person to group/followers (public is a group)

supported media

text
picture
video

hints are cool

let users know that they have pending messages

standard users have finite number of contemporaneous messages

certain number of private messages
certain number of public messages
all messages decay, especially public
finite number of access (time/accesses)
configure type of decay (number of accesses or time)

notifications

configurable
notifications hide themselves when you go out of radius
maybe timeout for notification hiding

security is a huge issue to consider

validate source of messages
you should be able to block people
high resolution filtering
warnings for public subscription
content checking for public submission ?

integration with existing social network ?

facebook
twitter
snapchat?
instagram
google

incentivization

upgrade to pro version if you get your friends to download the app
offer ads for free, users rewarded by company for buying app

users can rate other users public content

upvote kinda like reddit
filter public content by votes/relevance
certain number of likes per day

Why it is marketable

For users

ping for message within a certain radius

you get a compass to help you find you location

For businesses

local as well as corporate ads

enterprise users can have a web portal where they can customize their content they can push, user lists, user data

need customizable message creator and information infrastructure

promotional materials

Milestones

Initial beta release

drop a payload at a geographic location

pictures
videos
maybe like snapchat, put text on pictures

should be able to invite people

by phone number
see if friends have it (people in your contacts)

Gold release

Platinum release