Hmm, let's see...
A really BIG fireplace, the height and width of a regular wall, but a solid inch or two deep so it makes a very stable standalone piece. With a removable insert to make it count as two fireplaces back to back. Serves as a cool pass-through (with or without fire hazard), a good place to hide a secret door ("our situation has not improved"), or a portal transition piece between different DF sets ("what's 17th century France doing on a space ship?"). Make it flat on top so we can put minis up there.
Freestanding wall with chains on both sides, and a skeleton in the chains on one side. Standard wall or corner with a seated/slumped skeleton, or better yet a free-seated/slumped skeleton we can put anywhere in any set. Skeleton lying on the floor, sculpted on or free-lying. A skeleton "scattered" across a 1"x1" (or 2"x2"?) clear plastic base -- bones might extend beyond the edges of the base. (It seems like animated skeletons have been done to death, but skeletons as atmosphere pieces are in short supply.)
Spikes +1. Put them on one side of (half?) an elevation piece so we can walk them down a passage, or line them up as walls.
Wall height stone faces cut to fit in a standard (inside) corner, or a(n outside) corner junction of two straight walls. (Yes, just like the false alarms from "Labyrinth", but not legally actionable please.)
A big mound of coins cut into four unequal pieces. Put all four pieces together for a pile, put two together to go against a wall, or put one in each corner of a room. Simplest modular treasure pile.
More to come...
Favorite Pieces: Chasms, 45 Degree Passages, River & Lava Banks, Ledges, Large Curved Walls, Elevation Arch