In the WWDC launch keynote, Apple announced that iCloud will be upgraded to what they called "iCloud +" which includes various features designed to increase privacy such as Private Relay and Hide my email. Another feature included in iCloud + that wasn't mentioned is that it will also allow you to create your own domain name.
On Apple's iOS 15 new features page, in the iCloud + section:
custom Personalize your iCloud email address with your own domain name and invite your family members to use the same domain with their iCloud email accounts.
Essentially, this means that when iCloud + is available, Apple will allow users to completely change their iCloud email address. For example, if the current email account is email@example.com, it can be changed to firstname.lastname@example.org, removing the iCloud domain reference to give it a more personalized or professional look.
Additionally, Apple will allow users to invite family members to use the same domain name with their iCloud mail accounts.Although it has not been confirmed whether members will need to be within Family Sharing, it seems likely.
If you've used Microsoft 365, Google Workspace, and other email providers, this type of email account personalization has been around for a long time.