Extend core select widget

Please help with the following:

I need a new widget (just a slightly altered select) that can be setup from the UI by anyone. I have done the following:

1. Created a new custom module and enabled it.

2. Created a new widget.

<?php

namespace Drupal\filtered_select\Plugin\Field\FieldWidget;

use Drupal\Core\Field\FieldItemListInterface;
use Drupal\Core\Field\Plugin\Field\FieldWidget\OptionsSelectWidget;
use Drupal\Core\Form\FormStateInterface;

/**
 * Plugin implementation of the 'filtered_select' widget.
 *
 * @FieldWidget(
 *   id = "filtered_select",
 *   label = @Translation("Filtered Select"),
 *   field_types = {
 *      "entity_reference"
 *   },
 *   multiple_values = FALSE
 * )
 */
class FilteredSelectWidget extends OptionsSelectWidget {

  /**
   * {@inheritdoc}
   */
  public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
    $element = parent::formElement($items, $delta, $element, $form, $form_state);
    // Use the parent select functionality,
    // just switch the element.
    $element['#type'] = 'fselect';

    return $element;
  }

}

3. Created new element so I can switch the theme. (not sure if this is necessary)

<?php

namespace Drupal\filtered_select\Element;

use Drupal\Core\Render\Element\Select;

/**
 * Provides the filtered select element.
 * 
 * @FormElement("fselect")
 */
class FilteredSelect extends Select {

  /**
   * {@inheritdoc}
   */
  public function getInfo() {
    $info = parent::getInfo();

    // Theme switch.
    $info['#theme'] = 'fselect';

    return $info;
  }
}

4. It did not work otherwise so I also got a hook_theme up in my module file like this:

/**
 * Implements hook_theme().
 */
function filtered_select_theme($existing, $type, $theme, $path): array {
    return [
        'fselect' => [
            'render element' => 'element',
            'template' => 'fselect',
        ],
    ];
}
  

5. I just copied the template from core into my custom module and it works in the sense that it prints the right template, I have not yet altered this one, I just want to get the same old core functionality working under another name first.

Issue: My new render element seems to be missing the options array and I can't figure out why. I've done really basic overwrites and I cannot understand why it does not build the options array.

Twig Var Dumps:

--- THIS IS THE CORE SELECT ---

array:15 [▼
  "element" =>array:41 [▶]
  "theme_hook_original" => "select"
  "attributes" =>Drupal\Core\Template\Attribute {#7543 ▶}
  "title_attributes" =>Drupal\Core\Template\Attribute {#7932 ▶}
  "content_attributes" =>Drupal\Core\Template\Attribute {#7896 ▶}
  "title_prefix" => []
  "title_suffix" => []
  "db_is_active" =>true
  "is_admin" =>true
  "logged_in" =>true
  "user" =>Drupal\Core\Session\AccountProxy {#2354 ▶}
  "directory" => "core/themes/seven"
  "options" =>array:3 [▼0 =>array:4 [▶]
    1 =>array:3 [▶]
    2 =>array:3 [▶]
  ]
  "#cache" =>array:1 [▶]
  "theme_hook_suggestions" => []
]

--- THIS IS MY 'NEW' Filtered Select ---

array:14 [▼
  "element" =>array:41 [▶]
  "theme_hook_original" => "fselect"
  "attributes" =>Drupal\Core\Template\Attribute {#7543 ▶}
  "title_attributes" =>Drupal\Core\Template\Attribute {#8034 ▶}
  "content_attributes" =>Drupal\Core\Template\Attribute {#7708 ▶}
  "title_prefix" => []
  "title_suffix" => []
  "db_is_active" =>true
  "is_admin" =>true
  "logged_in" =>true
  "user" =>Drupal\Core\Session\AccountProxy {#2354 ▶}
  "directory" => "core/themes/seven"
  "#cache" =>array:1 [▶]
  "theme_hook_suggestions" => []
]

As one can see, I'm missing the 'options' array and I cannot understand why. Please advise.

You are missing template_preprocess_select(). You could copy the code to your own preprocess hook for fselect or, which might be a better idea, extend select as base hook and use double underscores select__fselect for the extended template name. See How do I define a custom html.html.twig file?