"Unlimited" Domains
To compete with Google Workspace you should give the higher priced plans "unlimited" domains. Workspace has a limit of 600 domains - even if you set the limit at 50 or 100 you're giving us more than the current limits of 10 or 15. This should particularly apply to Visionary customers who are currently limited to 10. I can't imagine the expense to Proton for additional custom domains is very high, the mechanisms are already built out.

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Rhipidate commented
I'm not a Visionary customer and only coming to Proton in March 2025 where the offerings are 3 domains for single user Unlimited, 3 for Duo and 3 for Family. Just three domains for single user Unlimited is incredibly restrictive and a block to migrating from other services. To only have three domains for Duo and Family where there are multiple people with existing domains makes these products effectively unusable.
As @Chris Barnes says, allowing many more domains on an account is probably little overhead and it may well make the suite of products much more attractive.