It’s very good question, and I guess the majority of us have this problem.
I wanted to do lot’s of animation practice with Pencil2D and I did not just want to animate 10 sec clips of this and that because in the end I wanted to have maybe a 2-4 minute short animation. I’d get the practice at the same time as I had created a bigger project out of it.
I just happened to listen to a humor podcast that had funny stories in them about 3 minutes long, they were set up like any fairy tale story but with some grown up humor twist to it. I thought that would be perfect to visualize, so I started to animate following that sound clip. And that’s what I’m doing now.
To create a story of your own is much harder of course but I was fed up trying to find the perfect story to do which resulted in no animation being done so that’s why I went for what I’m doing now.