OMG! I can't wait for Toady to implement tunnelling creatures other than dwarves! (Seen suggested
here by Name Lips, probably a million times over by other posters previously). How awesome it would be for Hairy McGairy to have just gone to bed after a hard days mining and drinking when all of a sudden a carnivorous giant worm bursts through the wall and devours Hairy McGairy and her three children! What joy it would be to have to decide a) seal up the network of tunnels the worm left behind? or b) send in some troops to explore the caves and chop down the worm's family tree?
Sorta gets a bit Dungeon Keeper on that one, but the havoc and surprise such new creatures could sow makes it's exciting all the same. It would certainly be different to building-destroying foes on the outside, or the slightly predictable imps and drakes that you know will turn up somewhere on that magma pool. It's extra nasty that, not only could tunnelers make ruin of your water ways and rooms, but when collapses and cave-ins are reinstated, they could cause some of the biggest destruction you'd ever see other than your own (picture that beautiful underground spire you carved out becoming dust as a single worm eats the pivotal support! Picture that heavy-traffic bridge bustling with dwarves, caravans and animals as a worm suddenly eats away the earth on one side! See the arm of that magnificent statue you carved break and crush the dwarves below as the termites vandalise your creation and make it into honeycomb!)
I would imagine that some worms would be heliophobic and may never breach the surface, while others would openly breach, explore and probably attack. Then they could also be divided into detivores or carnivores across this dimension. A special phenomena could be that the worms deposit a particular cast that could be used in alchemy, medicine, glass production or to add special qualities to metal. For giant worms the cast could be big enough to be a special material itself.
The more I've read about them (wiki), it's a wonder that worms are not incorporated more into dwarven lore!