To my mind, although requiring a store of information to call on ( you can’t imagine anything if you don’t already know and haven’t already experienced anything), imagination is not reaction to external stimuli. It isn’t so much ‘I think therefore I am’ but more ‘ I think about thinking and therefore I can imagine’. It is therefore linked to consciousness. It is the ability to think conceptually and is therefore also linked to intelligence.
Children cannot think conceptually until they reach a certain age. They cannot see things from anything other than their own perspective, both literally and metaphorically, until the brain reaches a certain stage of development. However some people’s ability to understand other people’s perspectives, to see issues from different angles and to ‘imagine’ novel solutions to problems remains very limited. To take a new situation and to apply existing knowledge in an entirely different context to solve a problem requires intelligence. Similarly taking a concept such as writing a fiction for example something like Lord Of The Rings requires the ability to take existing knowledge, apply it in an entirely different context and put it together to create a story. This is imagination in action.
In short therefore I would say that imagination is a mixture of consciousness and intelligence or at least, imagination is not possible without a measure of both.

I think imagination is seated in the fear of the unknown known. What made the thunderclap and the lightening bolt...what made the earth shake..etc.
Once the danger has passed the fear lingers which gave rise to inquisitiveness and then attempts to appeasement of the unseen forces that produced fearsome phenomena.
I believe things like hunger and thirst and need for warmth on a cold day are catalyst for stimulating prescient and resident and theretofore unused intelligence.
T Edison said genius is 1 percent inspiration and 99 percent perspiration. Trial and error along with good record keeping/memory, and scientific method bring repeatable results.New knowledge builds on known and consistent outcomes.
Observation of basic and natural phenomena like thunder and lightening brought a natural response of fear and anxiety. In the primitive mans (primitive knowledge) imagination kicked in naturally - it was primal self interest - self preservation that stimulated his imagination to devise ways to better survive a hostile environment.

You're gonna have to deemphasize touch. All of our senses, including the lizard brain responses to stimuli, roll into our experience of reality. Imagination is probably related to threat recognition, like when you catch a peripheral glimpse of a stick and jump back from the snake that your lizard brain dreamed up. The "snake" was a figment of your lizard brain's imagination. So, maybe imagination is a development of threat detection toward risk/benefit analysis. You sort of have to play with an idea of how lizard brain instinctual animal slowly developed social and domain complexities that caused individuals to have to dream shit up to explain discrepancies between prior assumptions and current experience.

I have discussed this before but forgive me for going over the same points. Imagination is not exclusive to humans. A deer hears rustling in the grass and assigns agency to the sound because it is better to be wrong and imagine a predator than be eaten. Humans assign agency to every thing because a world without agency from the animal perspective is incomprehensible. Civilization has to rise to near current levels before God or gods could be killed off.

Civilization didn't kill imagination it just shifted it to the abstract. The instincts didn't go away they have been interrupted from a different environmental perspective.

Humans are the cultural ape. We didn't start using tools because we have big brains we have big brains because our ancestors used tools. Tools are cultural adaptations they are not just variations from stones but include language. Language and imagination are abstract tools. Math is an abstract language tool. It is like science reductionistic. It simplifies by unnatural division and reduces the irreducible. The is no such thing as one because nothing exists independent of everything else but we cannot comprehend everything at once.

At the same time that math has enabled us to understand the universe it can produce what are probably the most useless of imaginary things such as string theory. When using abstract thinking tools we have to be careful to not let imagination run wild. We have to experiment to see what is real and what is not. Just like the deer has to test it's theory that the sound in the grass is a predator.

You comment about the connection between the senses and imagination is insightful. Take music for example. Music not only stimulates are senses but our imagination. Art and philosophy set us free temporarily from the restraints of experienced reality. It expands the set of the possible.

From a deeper perspective it is important to understand randomness. The existence of the universe perhaps and certainly life is dependent on what we call random events. To what degree there are indeterminate events is open to debate none the less we accept that some causes are unknowable especially in complex chaotic systems such as life. Pseudo random number generation is similarly indispensable to artificial intelligence and I would argue cognition. Animals and certainly human animals are not analogous to analogue machines. Without random thoughts we would have no thought only responses. Imagination is the key to intelligence.

I like the starting point.