This one reminds me a little of a Japanese noren - the half curtain that hangs over some doorways. In part, that may be because the color of the ink is close to the traditional blues you see in some Japanese textiles.
The angles of an envelope present an inviting angle to fill with doodles. Like the lines of a legal pad, the boundaries created by the folds provide a structure at the beginning.
There are times when it seems best to not take a doodle all the way to it's logical conclusion.
Once forms start to emerge in my doodling. I'm then feel the compulsion to color between the lines. Although shading would be a better term for it. These doodles were the result of another client call. You can see my notes: "People interacting with technology." Fascinating.