A 'billion dollar harem fanfic' might be a work of fan - created fiction where there is a harem situation. It could potentially involve a wealthy or powerful main character (maybe related to the 'billion dollar' part) who has a group of characters of the opposite sex (or various genders depending on the nature of the fanfic) attracted to them. However, this is just speculation.
Well, in a 'billion dollar harem fanfic', the 'billion dollar' could imply a high - stakes or luxurious setting. Maybe the main character is a billionaire and has a harem of people around them for various reasons. It could be about the relationships within this harem, the power dynamics, or the adventures that unfold with such a setup. Fanfics often take the basic concepts from the original source material and spin them in unique ways, so this one could be a really interesting take on the harem concept in a very wealthy context.