Experiment with any kind of imagery: pleasant, disgusting, exciting, wishful, nightmarish. Sit and think about everything and think in depth by topic or theme of choice: all from memory. The list of mental exercises are vast, so I’m not going to write you one but do sit and think or look at a image or video of anything and imagine more about it. For instance take Star Trek, look at USS Enterprise ship and think who build it, how, what materials where does materials where collected, what kind of problems arise from constructing the first ship capable of tachyon speeds and think anything else that you can about it, take any topic real or fantastical but try without limiting in doing so to think logical, also try to think absurdly as much as you think… create stories. This simple exercise can motivate your thinking and hopefully you can deduce what are you patterns of creative thinking (I know some of mind, to share them may not make any progress come to you or anyone else, tho, I think all of us think very similarly if not in the same way).
It’s also good to take a look at other people’s mnemonics for any type of data and try and figure why they come about those instead of anything else, also try to come up with a mnemonic image not necessarily to memorize the target idea, but for creativity arising purposes. Creativity is a type of imagination that does not focus on accurate recall of known ideas/images but the twisting, mixing and repurposing of known or remembered ideas. Creativity is the innovation that happens in the mind.
Check the following posts:
Abstract words
Tips for picturing abstract words
Brainstorming abstract words images
Imaginary palaces idea, good, bad, or indifferent?
Tips for making bizarre images
How do I create an Imaginary memory palace?
Other interesting but not necessarily about improving imagination:
Hyper Imagination - Method
Imaginary Memory Palaces
Self Imagination and Recall
Hyperphantasia