Use only one template to display product using a helper

getListItemHtml.php

<?php

class Namespace_Catalog_Helper_Data extends Mage_core_Helper_Abstract {

const BLOCK_TYPE = "namespace_catalog/product_list_item";
const TEMPLATE   = "catalog/product/list/item.phtml";

/**
     * Get product list item HTML
     *
     * @param Mage_Catalog_Model_Product $product
     * @param null|string $template
     *
     * @return mixed
     */
    public function getListItemHtml(Mage_Catalog_Model_Product $product, $template = null)
    {
        return Mage::app()->getLayout()
            ->createBlock(self::BLOCK_TYPE)
            ->setTemplate($template ? $template : self::TEMPLATE)
            ->setProduct($product)
            ->toHtml();
    }
    
}

item.php

<?php
/**
 * This file is part of Namespace_Catalog for Magento.
 *
 * @license All rights reserved
 * @author Phuong LE <phuong.le@agence-soon.fr> <@>
 * @category Namespace
 * @package Namespace_Catalog
 * @copyright Copyright (c) 2015 Agence-Soon (http://agence-soon.fr)
 */

/**
 * Product_List_Item Block
 * @package Namespace_Catalog
 */
class Namespace_Catalog_Block_Product_List_Item extends Mage_Catalog_Block_Product_Abstract
{
   //Add your additionnal methods
}

item.phtml

<?php
/** Build Your template for your item 
*   Make sure your class Block return your product
*   Here we have a basic example of template
**/
/** @var Namespace_Catalog_Block_Product_List_Item $this **/
$_product = $this->getProduct();
$_imageWidth = 200;
$_imageHeight = 200;
?>
<div class="product-list">
  
  <ul>
    <li><?php echo $_product->getName(); ?></li>
    <li><?php echo $_product->getDescription(); ?></li>
    <li><?php echo $_product->getPrice(); ?></li>
    <li><?php echo $_product->getMyCustomAttribute(); ?></li>
  </ul>
  
  <img src="<?php echo $this->helper('catalog/image')->init($_product,'image')->resize($_imageWidth , $_imageHeight);?>"

</div>

items.phtml

<?php
/** Make sure that your class Block return Items 
For example Mage_Catalog_Block_Product_List
**/

$_items = $this->getItems();
$_myHelper = Mage::helper('namespace/catalog');
?>
<div>
    <?php foreach ($_items as $_item) :?>
        <?php echo $_myHelper->getListItemHtml($_item); ?>
    <?php endforeach ;?>
</div>