Make evergreen content that is always relevant. These are characters, plants, rocks, natural landscapes, futurism and fantasy. It is clear that you need to make models of decent quality and set an adequate price. I do not advise you to deal with furniture, jewelry, cars (these categories are crowded), weapons (also crowded, and there are also restrictions on marketplaces due to the possibilities of 3D printing). Don't chase trends because they don't last long