I saw a different suggestion here that included later in it a suggestion to allow a writer to customize their own plotline in canvas, by defining the relationships between the two connected boxes. My proposal is that if you're going that way (I thumbed it up btw), you'll actually consider making it work more like a mind-map. This way you'll be able to also connect character cards to the relevant plotline cards, as well as - probably - allow to generate parallel plotlines that may converge, or just branch out to sub-plots. I'll drop an image to get an idea of what I'm imagining below. Note the color coding helps to keep track of where the plotlines diverge. With the aforementioned functionality, adding new boxes may generate texts based on several such boxes.