Shared SMTP domains
If one company owns two tenants, they should be able to share the same namespace between the two tenants - they can add the SMTP text records necessary in DNS, and they can set up SMTP transport rules to ensure that one tenant owns the domain and the other has it as internal relay.
In cases where two companies merge, this can be done simply in the on-premises world. It can also be done when a an Office 365 company buys a non-Office 365 company. It should also be available in multiple tenant scenarios.