Hi Aaron,
First thanks for the confirmation and letting us know it is in development. I have added my thoughts to how I would see this done (perhaps multiple ways)
Rather than going down the tracking extension path, perhaps there could a template layer (like field notes) that people can use. This would have a predefined schema but obviously allow users to extend if necessary. You might simply provide two layers - boundaries (polygon) and events (point) and allow users to symbolise them in a web map as they see fit. Some of our boundaries are not allowed to be seen by the field workers due to cultural reasons.
Perhaps within Field maps web app, you could open the web map configuration, and accompanying the offline areas section, there could be an area to define the layers or a secondary web map for geofencing. The data doesn't have to be part of the operational web map itself, but the data would be still be downloaded for offline use.
Obviously this needs to scale both ways too. We could add these layers in GeoEvent or Velocity to do on the fly analysis of our location, but we also need to continue providing offline geofence capabilities while offline. We are working in very remote areas of the world where there is no network connection, so even if there is an event (entering a zone) it would need to be recorded offline and synced later.
I hope this helps.
Cheers,
Gareth