It could be interesting to have an application that once launched would analyze the live video or audio of the smartphone and if it detects rhythms or variations (for example a flash that pulses • • • — — — • • • ), the application would translate it into text “SOS”.
The application would also have a writing mode where you could write and it would translate the text into flash pulses or audio sounds.
We could communicate in the night without wifi, without bluetooth and without network ^^
Another idea related to this one. We could imagine that we could have personalized morse codes created from encryption keys. So we could send private messages and nobody could read them with the application except the recipient because you use his public key ^^