It has been said that Art imitates Reality, or perhaps it was the other way around. Regardless, it's true. We borrow from real life to create our art. Even those of us writing in science fiction and fantasy. Everything we do is based in the mundane world we are trying so hard to escape.
Early science fiction didn't happen until science began to take a foothold in the world, giving us a basis on which to question and explore concepts, many of which later became reality in their own right: Jules Verne's submarine, Star Trek's Communicators/cell phones, H.G. Wells's Time Machine... oh, maybe not that one yet.
What comes most from our reality, is not the laws of nature, but the nature of humanity. Critics often claim that the best works of literature are ones that we as human beings can relate to, works where the characters feel the things we feel, react the way we would react, and even come from the kind of background that we ourselves carry in our backpack of id. Our stories are pieces of our history blended together and dyed with new colors, hidden among the worlds we create, that are, in turn, based on the one we live in.
The old adage advises us to "write what you know." Yet how could we do anything else? Martians are described as "little green men," we take our own bodies and alter them slightly to make something we claim is new. In fantasy, we have horses with horns, big lizards with bad breath, and men that become not something unknown, but merely a variant of what is familiar: bats, or wolves. Elves, goblins, giants, fairies, leprechauns, and dwarves, they are all variants on humanity's natural form. Even our ghosts and gods take human or familiar forms. The Greeks were all human forms, the Egyptians blended human bodies with animal heads, the Native American spirits were mostly animal forms that they knew.
Where then, is the fantastic that is not grounded in our reality? It is all borrowed from what we know. Shakespeare may have said that "there is nothing new under the sun" because he was frustrated with his inability to concoct something completely alien to our existence.
However, knowing this, I advise you not to frustrate yourself trying to figure out what hasn't been thought of, but to look around and figure out what you could borrow to enrich your story. If you are sick, make notes on how it feels to be sick, what you think of while sick, how your voice sounds. Be a people watcher, that observes the weird things people do, for we, truly, are likely some of the most fantastic beings in our world. How muich weirder can you get than us, really?