Bring back the old "Rounded Blocks"
I may get flamed for this but do you remember the original "Rounded Armor" blocks that were put in before the "Round Corners and Slopes".
I liked the idea, but at the time it was an unpopular substitute for round armor blocks that we have now. The blocks took their shapes based on the blocks they were attached to, allowing creation of many shapes and lines not possible with the standard blocks. I think part of the problem was that it was unclear exactly how they were supposed to work.
At the time, I thought it was a neat feature, and now that I've learned some modding, I think a lot could be done with them if their shapes could be changed with mods.