*Polygonal borders*.
That’d be real useful to structure text, IMO. Using only four-edged blocks, it’s hard to align text to some background-picture(having slanted edges), it can be done by using multiple small blocks(that float along the wall, increasing their width), but it is likely to break when resizing the text.
With polygonal borders, it should be easier to create a block that forms the text inside.
Or, joining blocks(though this doesn’t belong to borders), that’d achieve the same result by a different way.