Use case:
- Send an invitation with an attached map that shows how to get to your address.
- A flyer or other publication that shows where your business is at, along with directions from the nearest mass-transit station or highway exit.

The fact is that it's often harder to get around in a city than it is to make your way to the city in the first place and personal navigation devices have maps that get outdated quickly.

Implementation:
- Have the user enter a destination address (or use a push-pin on the slippy map)
- Have them select a train or bus station, or a highway exit, or another point of entry into the city.
- The rendering of this map is to be determined how best to tackle it, and at what stage (a different stylesheet and during the normal Mapnik stage, or using Cairo to overlay afterward, etc.)

Presentation:
- The map would have a bounding box who's covered area emphasizes the selected route, in effect a blow up (z=17 or z=18) of a specific part of the city.

Something along these lines would be useful to have at some point in the future. Feel free to brainstorm on what shape it could take, additional use cases, ways of presenting the map, and so on.