Eight wise monsters
And Doctor Who is the kind of setting where you could use all of them.
Alien invader? destroyer of worlds? Reptilian overlord? Sure.
Sentient virus? Robot uprising? Dark One? Absolutely.
(And the rogue robot advising people to follow their dreams, as it follows its dreams of taking over a planet, seems like the kind of mad detail that would suit such an adventure.)
Apocalyptic army of skeletons? It's been done.
Morrigan, Goddess of War and Death? Not much of a stretch, say she's from beyond the universe. She turned up in this week's 2000AD as patron goddess of the Iceni, in point of fact, so you can have Boudicca pop in as well.