What about special dens that have prisoner dwarves? So if you kill them, they join you, could even make them unique skins (like throws hammer instead of axe, or boomerang, instead of a runner it could be a naked gnome, or a packmule, that type of thing. Also could have blueprints that you obtain for unique single things, like special turrets or ways of upgrading a single unit. Love the weather ideas for sure. Could also have things that fly and if shot drop a bag of stuff, like gargoyles, drakes etc. And maybe things that run up and down the walls like in old arcade games where you kill the mothership, could make them a prestige upgrade. Could be cool if special levels are actual bosses, like an eye with a laser that shoots, and a mouth that eats falling gems etc. So like the mountain is an actual being of some sort. Others could include whipping tentacles near the lower part of the wall that knock runners back, and pustules that when popped rain acid down or something sticky that stops runners for X seconds. For weather theres also quick sand/mud that could leave puddles and if a runner touches it they are stuck for some time, but it vanishes quickly if no one touches it.
Sorry me again. Other ideas. Bosses that downgrade 1 random gem to an ore every second. Ones that half income while alive. Increase reload times while alive. So like debuffs while alive making them a threat on top of their actions. Projectiles that make runners walk in the opposite direction for X seconds when hit instead of stunned. An animal that sniffs ores and concerts them to gems or artifacts (very small chance for artifacts) id make it something silly like alligator.
More ideas: An airship building that has cannons like grapeshot, flaming, normal, etc. Maybe could have dwarves that throw hooks at projectiles and reel them in like fishing almost.
Laser could have an upgrade that splits the beam if it hits a falling ore
If something says it's benefits by scientists or whatever, it needs to say exactly how much by and then a total % currently in brackets.
Digger, scoops up a bucket full of gems and drives it back home, then needs refueling. Can be hit by falling stuff which causes it to lose part of the cargo. Upgrade to have turrets on it that shoot at falling stuff.
Breeding program, each house has an X% chance per minute to give a free dwarf. Make it small chance like 0.1% or something.
Driller. A dwarf with a road drill (one that points downwards), causes mini earthquake that shakes ores and gems off the piles (so they roll down)
Treasure floors, just fill the wall with special dens that hive artifacts and big gold.
You can drop artifacts that reward dwarves from dens, though it would be funny if runners could also explode from the dens and land on the ground and carry on collecting.
I have notes for demons/golbats or goblin flying machines that fly out and drop bombs on runners. Basically flying dens that can't be destroyed by normal attacks, but by turrets (but they must kinda always expire on their own, otherwise if player doesn't build a lot of turrets it could become too much).
I also have ideas for active "golden cookie" elements where eg "loot goblins" or wizards carrying gold bags, or UFOs fly out and if you click them fast enough you destroy them and they drop loot.
But yeah, lots of cool stuff is possible. I have a lot of similar stuff in my notes. Monsters on the wall are basically just moving dens or re-skinned dens that all fundamentally just stun/disrupt runners. So there is a lot of potential for those, though I must be sure they aren't too annoying/unfair for the player, since the current mechanics don't really allow for much counterplay, so most of these just end up being visual juice/flavor, which is still fine of course.