It is great that you are bringing back the vanity domain function for Outlook.com
When you discontinued the old 'Live Domain' personal email functionality, existing users were allowed to continue using the service.
This service allowed for more that 5 aliases to be created, and therefore those users cannot migrate to the new service without losing email.

Could there be a way to link multiple Office Home subscriptions to the same domain to allow sharing across families with more than 5 or 6 people?