I've recently been asked by a fellow colleague to assist him with an Office 365 re-architecture he is taking on. Currently there is one office in the United States and then one in Germany. The Office 365 tenant currently is associated to a single domain and all global users belong to this tenant.
However for the re-architecture they are changing to a new domain name. Due to the GDPR laws German data has to remain in Germany (or some other EU country) in it's own tenant. Naturally they don't want their US user base to reside in that tenant.
Based on my knowledge of Office 365 I advised him that due to the DNS configurations required for Office 365 that they can't span a single domain across multiple tenants. The best way to approach it would be to create their main domain and then additional domains in the same forest to apply to the tenants.
My knowledge is more about Azure AD and Exchange Online. So my question is if they go with using sub-domains for the additional tenants would they have to use those sub-domains for the other Office 365 services (In-Tune, SharePoint, Skype for Business, etc)? As well since I've never architected an AD domain at this level, this being my first real shot at it. What other considerations should be made before executing anything. Time is plentiful as they are in the planning phases but need suggestions soon to start prototyping some executions.