I reckon that the District line is of the maximum complexity that can be covered by a single line identity. The expanded London Overground is now way beyond that level of complexity, and is crying out for a range of line identities. And those identities should not just be the end points, that is what is wrong with how directions are indicated on the Paris Metro - you can determine the compass direction by only consulting the local part of a map, but to identify a line and/or direction by a destination requires viewing a lot more of the map.