The closely related Silurians and Sea Devils earn the mantle of recurring villains by the skin of their teeth – The Sea Devils counting (in my mind at least) as a sequel to The Silurians; Warriors of the Deep being the return that promised much but didn’t deliver, and the less said about the use of the Silurians in Nu Who the better. But excellent recurring villains they do make – not least because there is such ambiguity about whether it is appropriate to count them as baddies.
Functionally yes, the Silurians are not on the same side as mankind, and as with most species in Doctor Who they have members of their society who mistrust other species and whose response is to kill out of fear. But their debut appearance, and to a lesser extent subsequent appearances, demonstrated that they are also a moral and sophisticated civilization, whose main difference is their reptilian origins. I think it is the fact they are so similar, and yet so different, that makes them such a compelling adversary to face. Certainly the two Pertwee stories to feature a reptilian foe are excellent – and my regret is that when the Silurians returned in Matt Smith’s first season they did not retain the deep gravelly tones of the originals – they weren’t sufficiently different to humanity.
There are of course two avenues that could be explored further down the line, and I hope that one or both of them would be:
1: The Sea Devils return – Strictly speaking it doesn’t need to be the Sea Devils – it can be any reptilian species similar to the Silurians but different – hence the Sea Devils were modeled on turtles. Perhaps a species based on the Komodo Dragon?
2: We discover extraterrestrial Silurians – to a certain extent this has already happened – in Dinosaurs on a Spaceship (an episode I’m slightly embarrassed to say I rather enjoy) we discover that some Silurians escaped the earth before what they presumed would be their imminent demise. It’s intriguing to think if a Silurian colony on another world would have become peaceful or hostile.
I have a lot of affection for their stories – but I think the slight lack of threat means that the Silurians are more of an interesting enemy than a feared foe – which is why they are bottom of my enemies list.