Retrieve content of page by URL Parameter

There is a way, simply register your query:

function my_query($vars) {
    return array('your_query') + $vars;
}
add_filter('query_vars', 'my_query');

And get custom template the query exists (here /?your_query=first gets template called custom_template.php from your theme folder.

function my_query_template($template) {
  global $wp;
  if ($wp->query_vars['your_query']=='first') {
    return dirname( __FILE__ ) . '/custom_template.php';
  }
  else {
    return $template;
  }
}
add_filter('single_template', 'my_query_template');