I have built a cakePHP3 application and I'm looking for some advice concerning the following situation. An e-mail configuration is setup as follows, to send a mail whenever a new order has been created.
public function orderCreated($order, array $attachments = [])
    {
        $email = $this
            ->locale($order->customer->language)
            ->to($order->customer->email)
            ->from(Configure::read('Webshop.email'), Configure::read('Webshop.name'))
            ->subject(__('Confirmation of your order {0}', $order->nr))
            ->template('orders' . DS . 'order_created')
            ->set(['order' => $order])
            ->attachments($attachments);
        return $email;
    }
Has worked perfectly fine for ages, but I'd like to add some new functionality to this specific e-mail (and others). Admins should be able to override the content of the orders/order_created.ctp template if they want to, so they can define the content of this e-mail theirselves. So they don't have to rely on the same content in the order_created.ctp template I did provide.
Creating a UI for saving their own e-mails is not the issue. But the issue I don't really know how I could provide the overrided content to the cakePHP3 mailer. I tried setting
 ->template(false)
 ->message($new_content)
But that didn't help. Message doesn't reach mailbox because the body is empty.
Thanks.