What do You think about idea of remote control using another device in "master" mode?
Idea is that two navigators would be linked,
so both driver and pilot would work on same map.
Pilot could add and modify waypoints, change routing settings, zoom and view, block roads etc etc.
for offline it would have to happen over bt, but if it could work also over internet this would allow even more possibilities, like cooperating with someone who is roaming in the city, picking up passenger, or just calling a friend so he could help us navigating somewhere while we are driving - very usefull for driving in busy cities where driver cannot find even short time parking to change anything of the route.