Si-Fi + Detective. With no fantasy elements. No transmigration, no reincarnation, no time travelers, etc. Pure science fiction without space magic, warp, etc. Extraterrestrial species either absent or scarce, with no space orcs, elves, space bugs(Tyranids), etc.
Space opera + Adventure. Write a work similar to Cowboy Bebop, Star Trek.
Social science fiction. Without any additional things. Write about society, politics, etc. How certain inventions led to an alternate reality. How certain inventions led to achieving utopia\antiutopia. New faith changed the way the world operates. Sociology studies led to a different world structure and so on. Not a kingdom, state-building. No transmigration, no reincarnation, no time travelers, etc. No fantasy, no magic.
Ethnic fantasy. Write a story that is inspired by unpopular ethnic tales or from an unpopular culture\mythologies. Those who read novels, mangas, watch anime, and so on mostly like or at least heard of everything that came from Japan. Everything Japanesque is popular. Samurais, ninjas and so on. Different Japanese yokai, gods, fairy tales, legends, and so on, are also popular. Japanese mythology is popular. We have Romance of the Three Kingdoms from China. Tons of Chinese novels were inspired by their culture\mythology, and we read the translated versions. A lot of people like and know about northern European tales. About Vikings, Norse mythology, and so on. A lot of people know about Greece gods and tales. About Roman culture. Korean culture becomes more popular through manhwas and so on. There is a lot of books written in fantasy genres, historical genres, etc. All of them mostly use the default medieval European world. Without adding something original.
There aren't many games, movies, novels, and so on about Slavic culture, Aztec culture, African culture. Culture of India, cultures of different Island states. Culture of Steppe people and many more. Voodooism, shamanism, paganism. I suggest a theme to write a fantasy that embodies the spirit, the culture, the mythology, incorporates tales and legends from unpopular cultures.