There is a lot one could do offline. Highlight close by stations, have overlays/styles for transport focus (they got an overlay for "Subway", which in some areas includes commuter trains but not for bus/tram/light rail/...)

And in theory one could add bundles of data based on GTFS data which many transport organisations these days publish and do routing at least based on schedule times.

Yes, but that would be miles behind G/A maps, on top of being hard to implement.

E.g. Motis needs walk routing data that weights hundred of gigas.

Question is what it's for. A proper transport layer on the map would be valuable to me. Even without timetable data. That's even in relations on openstreetmap already.

Next improvement might be to highlight lines based on frequency of service. Still possible to precalculate with little need on device.

Routing ain't easy. That's true.

We'd love to have your input for improved transit in CoMaps! I agree it's an important priority. Last GSoC there was a project for ingesting GTFS feeds against OSM data, since local agencies are often much better about their details than OSM is, but as a student project it was slow and limited in scope.