Following up a bit on an earlier entry:
The Sontarans are your generic Proud Warrior Race. Except they look like potatoes, which is a little different. Really, they’re nothing interesting. Just interplanetary thugs. Nevertheless (presumably because of fandom’s Bob Holmes fetish) they hold one of the high chairs in the Doctor Who rogues gallery.
This is weird, since even in a plot sense (never mind a psychological one) they pose little fundamental threat. They’re focused on their own agenda, which may or may involve the protagonists or anything that matters to them. Furthermore, however narrow they are, Sontarans are rational beings — so in theory, a clever person might persuade them into some alternative plan of action. Any peril, then, is contextual, and tends to amount to “danger of being shot, maybe, if you’re unlucky enough to be in the way and someone is feeling grumpy”.
The only pregnant thing (pun intended) about the creatures is the thematic issue that, in theory, they are all clones. (In practice, no two have looked remotely alike; chalk that up to the costume department.) Though it’s hardly a novel subject, that’s maybe got some story potential — yet to date it’s just background detail, as is everything else about them, except that they like to stomp around and shoot ray guns at our heroes. Sometimes.
For a year now fans have whined about the Judoon, claiming they’re just lame rip-offs of the Sontarans. If anything, though, the Judoon hold more story potential, in that as police-for-hire they represent a more usefully gray area, and that their extraordinarily procedural minds can lead to some amusing behavior and twists of logic. Plus, what a great prosthetic! Though in some ways they do fill the Sontarans’ practical role, they do it far more successfully (and more originally), following their orders (for good or ill) in a ridiculously literal manner. Lots you can do with that!
Again, if a new series writer were to go into the cloning business, maybe there would be a bit of a point to the Sontarans. Otherwise, I’m not sure why they need to be explored. (My guess: that’s just what’s going to happen this year. We shall see!)