[Joomla!] - Extend core component xml fields via System Plugin

plugin_xml.php

<?php defined('_JEXEC') or die;
class plgSystemPlugin extends JPlugin {
  public function __construct(&$subject, $config = array()) {
    parent::__construct($subject, $config);
  }
  
  /**
   * @event onContentPrepareForm
   */
  function onContentPrepareForm($form, $data) {
    $doc    = JFactory::getDocument();
    $app    = JFactory::getApplication();
    $option = $app->input->get('option');
    $xml    = $this->params->get('xml');
    
    /**
     * Additional core fields
     */
    if($xml && $app->isAdmin()){
      $template = $app->getTemplate();
      
      // Cases for component types
      switch($option) {
        
        case 'com_content':
          // Load current article
          $article = JTable::getInstance('content'); 
          $article->load(JRequest::getInt('id'));
          
          // Get article params
          $_params = new JRegistry();
          $_params->loadString($article->attribs);
  
          // Load form based on layout
          JForm::addFormPath(__DIR__.'/forms/com_content');

          // If layout frontpage
          if($_params->get('article_layout') == 'website:frontpage') {
            $form->loadFile('frontpage', false);
          }else{
            $form->removeField('image_fulltext', 'images');
            $form->removeField('image_fulltext_alt', 'images');
            $form->removeField('float_fulltext', 'images');
            $form->removeField('image_fulltext_caption', 'images');
            $form->removeField('image_intro_caption', 'images');
            $form->loadFile('extend', true);
          }
          break;
          
        case 'com_categories':
          JForm::addFormPath(__DIR__.'/forms/com_categories');
          $form->loadFile('extend', false);
          break;
          
        case 'com_modules':
          JForm::addFormPath(__DIR__.'/forms/com_modules');
          $form->loadFile('extend', false);
          break;
          
        case 'com_menus':
          JForm::addFormPath(__DIR__.'/forms/com_menus');
          $form->removeField('page_heading', 'params');
          $form->removeField('show_page_heading', 'params');  
          $form->loadFile('item_component', true);          
          break;
      }
    }
  }
}