Here's some ideas I've had.
1) 45 degree bend elbow. essentially half the current 90% turn elbow corridor. This I know could be useful in fitting odd shaped rooms into leftover space, as well as maybe making better use of space.
2) a room that is like an art gallery. something for displaying murals. could be a round room or maybe various sized rectangles.. with lots of door openings close together. or even a long hallway-ish thing. Could be a 1 unit tall level version, and a 2 story version with a ramp to the upper level.
3) T2 silo variant, that lays flat (only 1 level tall) instead of the current 1 wide, 1 deep, 2 tall one. I have lots of places in my lair where a t2 silo would be perfect if it was 1 wide, 2 deep, 1 tall.
4) do some murals that look like doorways or windows looking "into" scenery from around Istaria complete with plants and such and a screenshot as a background.
5) ideas for some decorative chambers: A bathing chamber with a small pool, and a sunning chamber with light appearing to come in from above.