Including template in shortcodes

I’m not quite sure this is what you are trying to do, however, this is one way to use an external template file for your shortcode output.

This is your shortcode function (e.g., in functions.php):

function qt_shortcodez() {
    return include 'template/shortcode.php';
} // function qt_shortcodez

And that is the very output (i.e., shortcode.php):

<?php
ob_start();
?>
<strong>Hi there!</strong>
<p>
    This is an include test for shortcodes...
</p>
<?php
return ob_get_clean();