Create modern sites within Sharepoint site collection
When Sharepoint admins create modern Team Sites using the "Create
Site" option on this page:
It should create the modern Team Site under the master Sharepoint site collection like a traditional Sharepoint site and allow the modern site to be customized and managed like any other Sharepoint site, including back-ups and resource management. For any existing modern Team Sites created using "Create Site" there should be an admin option in O365 for admins to move the existing orphaned modern Team Sites under the master site collection.
Robert Caretta commented
+1 for this. If we are already within a modern team or communications site, we should be able to create additional modern subsites.