In the new Collector for iOS it would be great to provide the ability to remove the offline map from your device without removing the basemap as well. That way a basemap can be re-used on another map without being downloaded again. This feature existed in the Classic Collector app for iOS (https://doc.arcgis.com/en/collector-classic/ios/collect-data/offline-use.htm#ESRI_SECTION1_7FBD92F23...) and it is missed in the new app.