One of my favourite methods of pre-writing is word clouds (also known as mind maps, word nets, and clustering). I find that this is the best way for me to come up with subplots which are connected to the main idea without overshadowing it or being too unlikely. It also helps me if I have a very rough idea (such as a genre) and I want to narrow it down into a writable plot.
To start, take your idea and write it in the middle of a sheet of paper. Then, draw a cloud around it. This is what I like to affectionately call “the beginning”.
As you think about the beginning, write down all of the ideas you associate with it. Circle these ideas and connect them to the beginning with lines. (To see an example, look at the image down below.)
If you think of things associated with an idea but not the beginning, write those down and connect them to the idea. If you think of something associated with several ideas, connect it to all of the ideas you associate it with. It is also perfectly ok to start a new cloud from an idea if you find yourself running out of room on the original page or if one idea has more associations than all the others.
Continue making these clouds and connections until you have either run out of ideas or feel you have enough ideas to create an outline.