I know this is incredibly easy to to with a template, but for a new user it seems fairly daunting to start adding template sensors, so I propose a helper to invert binary sensors. Later this could potentially be expanded to creating entities for entity attributes for those cases where they’re still hanging around (the REST integration sensors can create a lot of those).
Simple example of where this is useful:
I create groups of various binary sensors (motion, door, etc.), but there are some rooms where the door being open should indicate a lack of occupancy and closed means it’s occupied. So now instead of being able to create a simple group for those rooms (as any of the sensors being on means occupied) I have to either create a template sensor for the door sensor, or a template sensor instead of the group.
This would be similar to the Switch as X helper in some ways.