I propose Feature-Policy adopt Content-Security-Policy concept and be able to define all default, example:

Feature-Policy: default 'none'
Feature-Policy: default 'none'; fullscreen 'self'

where similarity in CSP would be

Content-Security-Policy: default-src 'none'
Content-Security-Policy: default-src 'none'; img-src 'self'

Otherwise we would be stuck to define all features separately, and since the features list will grow more and more, without the global default it likely will harmful web.