I am impressed with the American Public Transportation Association (APTA) website. A huge mine of information, it is full of standard proposals, some from the early days (2005-2007). While GTFS has gained a worldwide acceptance, there is a huge difference between the two standards.
GTFS tried to solve a problem, a pain point. APTA is a body that unifies the public transportation companies across the US. The vast quantity of information in the APTA standards is more for the professional from a transit agency than for a third party IT company. Yet, I have found many things that are not yet included in the GTFS and that might be there.
The density model of pedestrians walking towards a bus station is a topic that deserves much attention. A good chunk of the time necessary for someone to commute from point A to point B is spent walking to or from a bus/subway station.
The various types of stations are taking into account the geometry of the surroundings. Median station are different from curb stations. Just talking into account the traffic light and time for it to go from red to green could help people better organize their trips. In other words, I get on board the bus that is at the station or I wait for the next one for a specific amount of time.
I will continue to read the APTA standards- and try to identify features that could or should be part of GTFS. I write should with caution. One feature cannot satisfy all needs and not all the features can be implemented with the same priority.