More "Murky Weather"

No.3 in this small series, under construction -Another idea for it -
And first thoughts toward No.4 -
Some of the scraps in my collection are less than an inch square, but useful nonetheless - in fact it's the smallest ones that are often the "bright spark" that animates the piece.

How many elements need to be repeated, or similar, for a series to be cohesive? Is it enough to have a theme, and have all the pieces quite different?