Hmm well you shouldn't write a book just because, you should write because there is a story you have you really want to tell (unless of course you're trying to get better at writing in general) and if you want to write a really epic story like Lord of The Rings it should be your idea not anyone else's but if all you need is a category there are a lot
Medieval, Time travel, fiction about non-fictional events....etc....etc

About Minecraft or other trending games.