Geoclue heading could be updated from latest GPS fix
Using ModemManager directly there is a way to use a GPS fix and retrieve the track direction using NMEA GPVTG (https://www.trimble.com/OEM_ReceiverHelp/V4.44/en/NMEA-0183messages_VTG.html), which has both true and magnetic. From another issue here, it appears that true is preferred for the "heading" and so that should probably be used in this case. I think that this will be very useful when there are limitations in compass sensors (see #146 (closed)) and GeoClue can get a GPS fix to determine the track direction.
However, it should be noted that the definition of "heading" in the GC Location interface is ambiguous in that it doesn't clarify whether it is the direction of travel or the angle of the device (ie. compass), which are two different things. If it is decided that the existing heading is only the going to be the direction that the device is oriented then this track direction information could likely go into a new field to reflect this.