It finally looks as if I've found a solution to my design problems. I finished the first of the new blocks today, and I'm pleased with it. Have a look -
In the top right hand corner is the absolute last piece of the (Hoffman?) butterfly fabric, which I re-embroidered a little.
The patch on the bottom left was originally a rather strident orange linen, so I toned it down with a piece of yellow lace, which I embroidered to define the pattern.
The three yellow flowers were originally white, until I painted them.
There's a lot of yellow in this block, and I think I'll continue that trend, as a way of unifying all the blocks.
This has already been a long journey of trial and error, and usually I'd be sick of the whole project by now. But the continuing challenges seem to keep me from getting bored, which is probably just as well, because at the current rate of progress, I estimate it will take me about six months to finish.