Now I just need to find a sub-plot.
These are usually lying dormant within your very manuscript. Here's what happened to me...
In my 2nd novel (a private detective novel in a series I'm writing), I had a brief, 1-chapter encounter with a serial killer, "The Slicer". I had written this principally to show the police in action, and to plant the seed for a subsequent novel in the series.
I simply read thru this chapter, and thought about it. I asked myself "What happens if the killer strikes again? How does the detective react?"
And then came the critical thoughts: "How might my private detective protagonist get involved? How might he link up with the cops in a logical way?"
And it was while pondering this, I evolved a small sub-plot into a strong B-plot series of chapters that fell quickly into place.
In other words, just giving some careful thought to this original single chapter resulted in the addition of 15,000 new words.
So it's all there, festering in your fevered brain! ha ha