10 Doctor Who Villains You're Surprised Only Appeared Once
4. The Wirrn
Steven Moffat and Russell T Davies have both sang the praises of 'The Ark in Space' over the years, so it's odd that there's never been a sequel. Perhaps they felt the pressure of following a story that has such a lauded position in the canon. It's a shame because the Wirrn are suitably horrifying creations that could be realised to great effect with modern VFX.
The Wirrn are human-sized insect creatures that breed by using "cattle" species to grow their eggs. In 'The Ark in Space', this is the crew of the Nerva Beacon and is portrayed to horrifying effect with some of the most creative use of green bubble wrap that you'll ever see. It's so effective that a recent Bruce Willis film called 'Breach' has practically the same plot couldn't come close to matching the horror. Despite the slightly more advanced effects work and a higher age rating.
Maybe this is key to why the Wirrn haven't yet returned to the series, as any Doctor Who production team is wary of recreating the alchemy of those early Tom Baker stories. Regardless, these creepy crawlies are long overdue a return.