GeoJSON in GTFS? (Or the future of GTFS serialisation)

It is currently being proposed in GTFS-Flex (#388) to introduce a new serialisation format we have never worked with in GTFS, or GTFS-RT. In the past these kind of discussions (think zeromq, mqtt, websockets...) were explicitly _avoided_ in favor of _proven technology_. With the suggestion for a new serialisation format for GTFS-Flex namely: GeoJSON this feels to me as "too fast" especially because we historically have had this discussion about shapes.txt. But also more recently how to serialise more complex structures where CSV (honestly) also does not make sense either.

No, I am not proposing a full overhaul of all the files (or suggesting a well thought out format as NeTEx ;). But I think we can all agree if we bring in anything else than CSV it better be the _right_ solution and apply it to multiple places, not just for GTFS-Flex.

I already have asked @isabelle-dr if we could have some sort of meeting on this topic.