I like this plugin, but I don't want the signup form to be removed after the user signs up. The footer area in my theme just looks too empty without something there to visually fill it. I also saw a post from another user who wanted to set the expiration of the cookie.
Both of those could be accomplished if you added an option and/or filter to the expiration value in setcookie(). For example
$expiration = apply_filters( 'mcw_cookie_expiration', time() + 31556926 ); setcookie($this->id_base . '-' . $this->number, $this->hash_mailing_list_id(), $expiration);
I'd encourage you to add filters and actions throughout your plugin; otherwise it's impossible for users to modify or customize the plugin behavior without forking it or modifying core files that will get overwritten on upgrade.