As far as Orwell goes it isn't just "imagination" that led to the vision of state that is presented in 1984 but imagination based on extrapolation of experience. Orwell was an international reporter, combatant and traveler that saw and chronicled and even battled fascism in Spain, Germany, Italy and Russia among other nations and understood the inner workings of these repressive states. He was able to write about them with real strength in 1984 and other works.
I have read nearly all of Orwell over the years and a few biographies. Fascinating thinker and writer.