The phrase "overactive imagination" means having an imagination that is very active or lively, often leading to excessive daydreaming or making up unrealistic scenarios in your mind. Full definition
Book Fox (vulpes libris): small bibliovorous mammal of overactive imagination and uncommonly large bookshop expenses. (blog.feedspot.com)
He's not an amorphous bogeyman ready to leap from the shadows, but a piece of teenage folklore that by all rights shouldn't exist, brought to murderous life by overactive imaginations. (filmfreakcentral.net)
It appears that someone at Walmart Canada has a very overactive imagination or pressed a button too early, as tons of highly demanded sequels and games are suddenly listed on the website. (thenerdmag.com)