Adding a new required input field or making a previously optional input field now required, would break existing Zaps without a value for the field. There is no automated check for this when migrating so though it may seem possible, it will be a breaking change for migrated users.
Add new input fields as optional. Use a combination of custom error handling to throw an error if the field is empty and help text to explain the field is required.
Use Code Mode (Platform UI) or code in Perform (Platform CLI) to set a default value for the inputField if users have it blank in their existing Zaps. This would only be successful if a universal default could apply to all users (would not work for custom fields for example).
Create a new trigger/action/search (T/A/S) with the required inputField; and hide the previous T/A/S. That way, existing Zaps will continue to work with the previous (and now hidden) trigger/action/search definition, and new Zaps will be forced to provide a value for the required field.