A quality maps app seems like one of those audacious undertakings given how much there are to them. Sure, sources like OSM help quite a bit, but the amount of data needed (be it through one's own data acquisition, partnering, licensing, etc) is pretty massive. This is one of the main reasons the iOS maps had problems.

There are already niche apps for things like public transport, though you may need an app per city/region. Even those niche apps aren't always accurate -- the NYC transit apps (2 or 3 different once) I tried while there earlier this year didn't pick up on station closures due to maintenance.

Maps/transit routing/etc is an interesting problem, just one that seems difficult to do right as a startup (on the scale of ios or google maps).