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.