Perhaps of more general use, what to do with monsters that haven’t put in an appearance? Reviving a weird or obscure race, and bringing them up to Big Two-Parter status at least, is certainly tempting.
Go a bit Deep One and a bit The Fog with them. Rising out of the water on foggy nights, draped in seaweed, attacking coastal towns and too-close oilrigs, armed with tridents, disguising themselves with sou’westers and the like. Yes, I’ve run this episode.
Big green lizard-men with a colony on Mars. Who are actually pretty sociable as long as they aren’t at war with you. I’d probably drop the thick pincers, and I’d naturally want to see what’s under the helmets. And maybe give them different weapons? Ice axes seem like a sensible start...
Rutans or Zygons
(Not counting The Gunpowder Plot for the sake of this lineup.)
Shapeshifters with an agenda which isn’t directly inimical but no interest in preserving human life. A Secret Invasion in a small, sleepy town perhaps. But with an added threat of taking it to the heart of the nation.
Greeting Earth neighbours! We are the Axon and we’re here to help! Oh, you’re not doing anything important with all that crude oil, are you?
(I’ll get to them one of these years.) Created with one driving purpose. They will destroy the Daleks. And they don’t care about who gets in their way. They could stay as androids, be humans or aliens, but androids suits their unstoppable Terminator attitude to things. (If the Movellans ever show up, you’ve got two problems. One, you’ll be bloody lucky if you can get on their good side, because if you can they’ll help you fight the Daleks. Two, if they’re here at all, that probably means DALEKS!)
Crystal intelligences building or taking over robot bodies, threatening a future Earth colony? Hey, why not? Heck, give them swords!