How can I return the result of my custom function?

Seems pretty much simple:

add_filter( 'get_comment_author_link', 'attach_city_to_author' );
function attach_city_to_author( $author ) {
    $city = get_comment_meta( get_comment_ID(), 'city', true );
    if ( $city ) $author = $city;
    return $author;
}

Unless if you are adding the city name somewhere else and wanted to remove comment author name then call add_filter('get_comment_author', '__return_false');

Hope that helps.