Another way to make the plot sing is by mirroring. Characters can be mirrored, so can sub-plots, scenes and places. I've always thought of the god Janus--the one who looks both ways. Using this device has worked for a number of writers and you might consider doing this to add some spice to your life.
Take two characters, make one good and the other bad but give them the same motivations and goals. They will be very similar but so very different. These characters can also be from opposite backgrounds--rich/poor. city/country, sophisticates/naive. There are hundreds of variations that can be developed. Using two mirroring characters that show different ends to their lives or stories can bring home points the writer wants to make.
What about mirror sub-plots? This makes for a long book and can't be done in a short space. Having two stories in one takes words and development. This also can fall apart when one of the stories interest the writer more than the other. I've done this perhaps once and making the story turn out was very hard and came up to about 100,000 words. There must be a balance between the two stories and if you have more than two mirrored stories each segment must be written and woven into the other.
So mirroring is a way to add depth to your characters and add spice to the story. Just remember balance is what is needed and the moment one of the mirrors dominates the story becomes fractured.