I'd like to see a race that tame giant worm creature to dig them passages underground, which they use to invade forts/settlements/caves. Giant purring maggots?
If it's giant, chitinous, travels underground, and is fearsome enough to be worshipped, I want to see my dwarves conquer it...
I actually modded a race like that into my game. The worms, not the race that tames them. Of course, they enslaved all the dwarves rather than the other way around - Of course, my giant worms with tentacle faces didn't do much more with their tunnels than current races do, (aka nothing) but they did like enslaving dwarves, thats for sure. That was actually I pretty fun mod, since they were carnivorous which definitely changes gameplay, I should probably put it up sometime...
Err, anyways, on topic. I'm really hoping to see the complete removal of "underground rivers" and instead allow rivers to actually go underground (especially in limestone regions and the like) and flow that way before bursting out forther downhill. It will probably be a while before boats are in though, so I suppose if people wan't them navigable that wouldn't actually work.
It's good to know we'll finally be getting endless (or at least longer duration) sieges from chasms and the like, but I wonder if it will be different than currently, where that would just mean they'll keep accumulating on the lowest cliff and never be able to reach me.
Hmmm... I wonder if HFS will be a "world spanning layer"...
Pierce it anywhere, and you let out ALL the demons in the world! Now THERE'S an endgame. Especially if your adventurer can descend into the hole to explore this new world spanning layer.