I always wanted a (relatively) simple mobile application to navigate public transportation in different cities. Basically an interactive map for public transport. They'd be easy to read, easy to understand how connections work, etc. Every city (or even every agency within a city) uses different, and often horrible, diagrams of their lines, like MUNI vs BART in San Francisco.
This is 90% designers problem. The engineering side of this is easy. Hard to say how you'd pay money for it, but if your maps are good, some cities may agree to pay to officially use them.
Google maps kind of does this for you (if you do "directions") but I just don't understand why it doesn't work... It gets me to a subway station and I start searching for the maps on walls.
But if done well, millions of people will be thanking you every day.