public function viewElements(FieldItemListInterface $items, $langcode) {
    $elements = [];
    foreach ($items as $delta => $item) {
      $elements[$delta] = [
        '#theme' => 'my_field',
        '#type' => 'markup',
        // etc...
      ];
    }

    return $elements;
  }

function my_field_theme() {
  return array(
    'location_field' => array(
      'variables' => array(
        'x' => NULL,
        'y' => NULL,
        'z' => NULL,
      ),
      'template' => 'my_field',
    ),
  );
}

<section class="class">
  <h2>name</h2>
  <div class="cls" data-x="x" data-y="y" data-z="z">content</div>
</section>