It would be great if users were able to upload custom spatial files stored on the device to an offline area in Field Maps. It wouldn't need to be pushed to anywhere after the fact, just be able to be referenced on the local device. These could be Shapefiles, a File Geodatabase, a KML/KMZ, or even a CSV with Lat/Longs.
For example, a user has a Shapefile showing some points that they want to navigate to. The data is only required for this one user and is only required on this one tablet for a particular offline area that they are using. The user can navigate to the Shapefile stored on the device and upload it to that offline area for viewing. The data wouldn't be needed for any other map/offline area and would not need to be pushed back to Portal/AGOL.
If editing capability was also included then it would be good to export back to a local copy of the data after edits have been made. Editing functionality is not a must though.