GTFS sensor: expose more events

Currently, the GTFS sensor exposes only the delay until the next passage at the specified stop. I’m using it to notify me 12 minutes before the bus leaves the stop on a specific time-frame (I don’t have to catch a specific bus, but I want to know how I should leave to get the next one). However, depending on the time of the day, those bus are more frequent than one in 12 minutes, so the sensor never gets to 12.

I don’t know exactly how this could be exposed to HA automation, but firing an event x minutes before the next passage (regardless of the bus frequency) would be a nice feature to have.