I'm thinking of writing a sci-fi set in 400 BC (historically accurate) and it has subtle ancient alien theme to it (no direct aliens involved in the story tho). Which sci-fi sub-categories does it actually fall under? I've never encountered a story like it. It will have a lot of action, so maybe an action-adventure sci-fi? Adventure Alien Invasion Alternate History Anthologies Colonization Cyberpunk Dystopian Exploration First Contact Galactic Empire Genetic Engineering Hard Science Fiction History & Criticism Military Post-Apocalyptic Short Stories Space Opera Steampunk Time Travel
Honestly, if you're only thinking of writing it at this point - haven't started, much less finished, much less started needing to think about marketing - I wouldn't worry about what genre it is. I find that that's only a way to pigeonhole yourself into trying to write to a certain genre's conventions before you've even decided what the story's going to be. That said, are you sure it's even sci-fi? Historically accurate with only subtle ancient alien stuff? I mean, maybe alternate history, though my very incomplete impression of it wouldn't include things like what you've described. Without really knowing anything about it beyond that, though, it's difficult to offer any help. I'd advise just writing the story.
