Custom template redirect based on url vars

You’ll have to exit afterwards.

add_filter( 'template_redirect', 'wpse85164_query_var_template' );
function wpse85164_query_var_template( $template )
{
    if ( 'basic' === get_query_var( 'template' ) )
        exit( dirname( __FILE__ ).'/basic.php' );

    return $template;
}