Today, Sci-fi novels are some of the most popular, but was there an equivalent to this back in the times of the Greeks or the Romans? Did any ancient authors build ever worlds and tell tales filled with fascinating new technology that would have seemed as impossible to them as much of space is to us?
If fantasy novels, authors often take us to the far past of castles, lords, and peasants-- did ancient authors do something similar to this? Have 15th century British authors ever looked back and written novels based in the Roman era?
I hope that's clear. :) Essentially I'm asking if the sci-fi and modern fantasy genres would be similar to something that ancient readers would read? :)
You may be interested in the 'What was science fiction like in the past? How did ancient people imagine the future?' section of the Popular Questions pages.
Sci-fi has certainly been a thing in the past: the most famous example in classical literature would be the True History.
Fiction based off of the distant past has been a thing for just as long, if not longer: Homer's works (probably written in the 700sBCE) are set in the Mycenean period -- and, indeed, there is quite a lot of evidence that period details which would not have survived until Homer's time were accurate. Indeed, there is debate still as to whether or not the Trojan war was loosely based on a war which actually happened.