bbPress - Random Single Topic - http://bbpress.org/forums/topic/random-topic-link/

ntwb_bbpress_random_single_topic.php

<?php
/*
Plugin Name: bbPress - Random Single Topic
Plugin URI: https://gist.github.com/ntwb/ca87b9b75a42215b2889
Description: bbPress - Random Single Topic
Version: 0.1
Author: Stephen Edgar - Netweb
Author URI: http://netweb.com.au
*/
// bbPress - Random Single Topic
function ntwb_bbpress_random_single_topic() {
    if ( bbp_has_topics( array( 'orderby' => 'rand', 'posts_per_page' => 1 ) ) ) {

        while ( bbp_topics() ) : bbp_the_topic();

            ?>
            <a class="bbp-topic-permalink" href="<?php bbp_topic_permalink(); ?>"><?php bbp_topic_title(); ?></a>
            <?php

        endwhile;
    }
}
// Hook into action
add_action('bbp_template_before_lead_topic','ntwb_bbpress_random_single_topic');
// Add it as a shortcode [ntwb-bbpress-random-single-topic]
add_shortcode('ntwb-bbpress-random-single-topic', 'ntwb_bbpress_random_single_topic');