Thanks for your feedback! Just a note to let you know that we’re building your suggestion now. We plan to increase availability to this feature as we continue improving the tech, and ensuring a smooth experience across Office 365 following the URL changes in SharePoint Online.Richard commented
In addition to changing an existing sharepoint site domain, we should also be able to create several sharepoint sites under several different domain names just like we are able to create user accounts with any email address as long as that email address' domain has been verified to be owned by us.Richard commented
Come on and do this already - it was such a major oversight not to have added it to begin with. When I go to add a new site, the primary domain field is a DROPDOWN indicating that the functionality is already there, but it isn't - Please add this already!