This is really beautiful! The stone-brick pattern is great. The colors, too -- nice earthy grey that really makes it clear those are stone and not just grey-colored (clay) bricks.
If you weren't planning to add a foreground version of the columns, I'd recommend that, especially if the foreground version is fullwidth.
Rather than get rid of the diagonal background tiles, my suggestion would be to make diagonal foreground versions. More versatility in a tileset is always better.
Of course, if you don't need any of those things for your own level, you can always just leave it to other users to fill in the blank areas in the tileset...