A chat app, which gives you X amount of characters to send. The length of the message is deducted from X when you send a message and you need to move around in the real world at walking/running speeds to replenish the characters.

I think this would actually be better as a forum. Flame wars would get you fit!

A chat app that only lets you talk to people at the same altitude.

A chat app that only lets you talk to people on the other side of the planet.

A chat app that forces you to speak in any other language besides your mother tongue.

A chat app that Google Translates all your messages through five languages and back to the original.

A chat app that routes your messages to Intercom chat widget on a random website.