It is currently possible to implement an iFrame as a modal, but not inline. This makes a blended card experience less user-friendly. Ideally, a user would be able to view the iFrame at the same time as other UI components. This would allow a user to view externally hosted data that is not available through API and still interact with other UI components to take UI or CRM actions. Opening a modal requires context switching between the external platform and taking related actions.