'Fantasy' is somewhat of a sticky term; mostly in that it requires there to be major elements or technologies in the story line that aren't currently realized, aren't realizable by current understandings, and are kinda just thrown out there ad hoc. When I had a science fiction lit class back in high school the way the teacher explained it is that science fiction would be movies like, say, 1984 or possibly something like Star Trek, while Science Fantasy would be something like Star Wars (and essentially with the excuses made for Jedi capabilities its non-magic fantasy).
I think what we're left with, outside of inferring wild alien races and technologies, are the 'punk' genres - steampunk, dieselpunk, etc., where you try to unfold a different course of innovation and imagine what would have happened if we had been in a different world under different physical laws which made steam the way to go or something other than electricity. If you try to write a story, say, about the middle ages and its a fictional account but includes no magic, no goblins, no fairies, and the closest thing to magic is con-artistry perhaps - you don't have fantasy, rather you have something more like general fiction or historical fiction.
I'm not saying don't go for it if you have an idea, just understand as well that the term 'fantasy' is just a brittle classification, a product of language, and if your ideas end up being classified as fiction but fantasy inspired - nothing wrong with that!
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