Remember those fiery monsters in Fires of Pompeii from series 4?

I think they should be brought back to a future storyline with Capaldi. They have the opportunity to become iconic and recurring.

Also want to see the TV debut of the Charrl (from the books). They would be suitably creepy for the Capaldi era.

They were pretty good as monsters go, but if they returned i'd much rather it be in an episode set on pyrovilia than on earth, because, apart from the fact I want to see an alien planet with a native alien species anyway, there's the element that I think if the pyroviles were on earth again, they'd just be trying to achieve the same sort of thing as they were in that episode every time.

I like the episode and really should do a rewatch as haven't seen it since Peter C's casting as the Doctor, but I think a walking lump of fiery rock has a rather limited potential for development, tbh.