Developers have a property in AL named ExtendedDataType (https://docs.microsoft.com/en-us/dynamics365/business-central/dev-itpro/developer/properties/devenv-extendeddatatype-property).

In the modern client, this property does not really surface in the UI for URL and EMail, especially when the field is editable.

My suggestion if this one: if there is no lookup or drilldown specified for the field, then display a default button (similar to the one that was displayed in the retired Windows Client).
=> If the ExtendedDatatype is URL, run a Hyperlink command.
=> If the ExtendedDatatype is EMail, then, now we have a very good Email module, open an empty e-mail.

If on the top of that we could get global an event to catch the click with the current record, fieldno and field value to implement our own behavior, that would be awesome.