All ideas my mind wishes had enough time to build (not in order)
Crowd-sourced story telling
Any user can either start a new story or continue an existing one. Users will be able to upvote a story contribution. Two modes will exist: Chaos and linear.
In chaos mode everybody can continue at any time and at any point of the story. The story will then look like an upside-down tree with the story branching off in many different versions. The best story can then be sorted by calculating the path with highest cumulative upvote score.
In linear mode an user who attempts to continue the story will lock the entire story, write its continuation and unlock the story. No branching will be allowed.
I imagine a set of rules can be enforced by the story creator (e.g. maximum length per contribution).
Some stories may be geo-locked. The user can write a story (contribution) only if he/she is physically writing from the location selected.
Car stats and analysis
People have already done this. You buy a OBD reader, hook it up to your car and on the other end to a rasberry pi. Then you can install node on it and start thinkering with it. Show throttle opening and closing, show ideal speed to maximize gas consumption, collect stats on higher fuel consumptations based on hour of the day/temperature of the day. Collect start and sync them over wifi when parked near your house. Possibilities are endless. For example you can go as far as adding radar sensors in front and back to show distance from vehicles around you and their acceleration compared to the vehicle the system is installed on. If the distance becomes too low too quickly (car in front suddenly breaks) beep loudly, etc..
A crowd-sourced app to notify the city of various issues like potholes, water leakage, parking abuse, etc.. . There are similar solutions out there (a true competitor analysis hasn't been done) but the key of this app would be its simplicity. No more than 3 click will be necessary to report the city of a water leakage/pothole/etc. Another key component will be the Social aspect. From my personal experience people like to shame the city administration by reporting these things on the city's facebook/twitter page. If they could do it through Fosso it will also provide the city with a GPS location, a picture and other info.
I can imagine how a city-specific app will be built, so that the city employees can mark an issue as resolved (e.g. leakage fixed). People can subscribe to issues, up vote them, instant share.
A platform game that uses website's HTML to uniquevocaly generate maps the user can navigate and explore.