Allow when creating a field in both contacts and opportunities, to define if these are mandatory fields, this gives versatility to the system, and ensures that the required fields are filled out for correct processes
As an additional note, it would be excellent if a mandatory field could be requested through some function or workflow so that at some point it could be requested, for example
When changing a contact from Lead to Customer, make the TAX ID mandatory, this would help a lot to improve many filling processes