How to clone any standard webform element
The WebformElement plugin acts as a wrapper around a Drupal Form element plugin.  Therefore to actually create a webform element you need to define a Form element plugin with a corresponding Webform element plugin.

If you want to extend the Email element you need to extend the below classes


\Drupal\Core\Render\Element\Email
\Drupal\webform\Plugin\WebformElement\Email


All you might missing is a \Drupal\custom_module\Element\CustomModuleEmail class which probably will look something like...

&lt;?php

namespace Drupal\custom_module\Element;

use Drupal\Core\Render\Element\Email;

/**
 * @FormElement("custom_moduel_email")
 */
class CustomModuleEmail extends Email { }