I think one of the most important things for a big imagination is being curious. Those people tend to build a large pool of experiences. When you try a new thing or take a trip somewhere, or just talk to someone else about their life, you're exposed to fresh perspectives and inspiration that might get pulled out into a new illustration or story. A trip to Italy might get you thinking about a city full of waterways. How might that look in the morning? For the movie Cars the Pixar crew took a real road trip down Route 66 and talked to a multitude of locals to get inspiration for their sets and characters.
Even just reading a lot and learning about a new topic will earn you tidbits for future ideas. At some point you will take two ideas and mash them into something new. I've started keeping a notebook of interesting things I see/hear/think, to look back on later. A couple highlights:
- How do you fit a piano into your carry-on bag?
- Heard on Planet Earth 2: "We welcomed the smell of sulphur from the volcano because it covered up the smell of the penguins."
- What if the Dire Wolf was actually a Dour Wolf?
I also recommend picking up some graphic novels. I've always been fascinated by movie storyboards, and comics are a fancy version of those - a movie of still pictures. You start seeing how to incorporate the establishing shot, or the full page reveal, all sorts of storytelling tricks.