What algorithms model the imagination in agent or multi-agent systems?
I can’t say specifically what algorithms, though would also like to know. I can say that imagination in agent or multi-agent systems cuts two ways. There is a whole field dedicated to Computational creativity; however, I find Computational Dreaming [1], Daydreaming Machines [2], and Robot Dreams [3] more intriguing. See my recent answer to: How are metaphors handled in AI / NLP / ML?
[1] Computational Dreaming (Draft) | Meta-Guide.com
[2] Daydreaming Machines | Meta-Guide.com
[3] Robot Dreams | Meta-Guide.com