wp_dropdown_pages : choose post status to list in dropdown

If you look at the source for the function, you can see that the arguments to the function are passed directly through to get_pages(). That function supports a post_status argument that can be an array of statuses. So to do what you want all you need to do is pass the post statuses you want into a post_status argument of wp_dropdown_pages():

wp_dropdown_pages( arrray(
    'post_status' => array( 'future', 'publish' ),
) );