Many industries have data residency requirements that require their client data to stay within a certain country or under the control of the business. Often for compliance reasons.

Many of these industries would be VERY interested in using Proton services.

My suggestion would be to offer a self-hosted option for business accounts.

As for implementation, I would suggest specifying a single or multiple locations for each service at the account level and then being able to select on a user basis (or by user groups) where the data that user generates resides.