LOVE the ability to edit related table data using ArcGIS Online and Collector. My suggestion... it would be nice to have a different add/edit/delete permissions for the feature class vs the relate table.
My example: We have our parcel map displayed in Collector with mobile users adding records for garbage and recycling carts at each property in a related table (one parcel, many carts). We'd like the users to be able to edit parcel attribute data, but not modify the parcel geography (specifically, delete a parcel). But, if we restrict editing to only attribute data, the user cannot add new records to the relate table.