It would be useful to have the ability to add additional fields for improved symbology and labeling of the track layers. This could be done either by allowing the addition of new permanent fields in the track layer and then adding support for attribute rules to update them in real time or by adding support for table joins for real time value lookup. A use case would be to label last known location points of public safety personnel by call sign and then symbolize them by type (fire, police, etc).
I am aware that ArcGIS Online does not currently support attribute rules, but there is an idea page for it (see link below). Table joins to the track layer could be done in GeoEvent server (or possibly ArcGIS Analytics for IoT), but this is cost prohibitive and the only desired functionality from those products is the lookup functionality needed to enrich the data.