There is a great blog post here by @BernSzukalski showing how to create a custom basemap gallery:
https://www.esri.com/arcgis-blog/products/arcgis-online/mapping/custom-Basemap-gallery/
However I wondered if anyone knew if it was possible to have both a custom basemap gallery AND the ESRI default gallery available as standard? I'd like to add a couple of new basemaps to make them available within the organisation, but I don't want to lose all of the fabulous default basemaps. I could recreate the basemaps that ESRI have as default, but if a new one appears I won't know about it!
Many thanks for any thoughts,
Helen