I like to build off of mythology and existing ideas. Not for the sake of stealing them, but because in a strange way, they feel more real than something I could come up with myself. For example, vampires. Vampires are something that the majority of the world has an idea about. It's part of our culture, it's something that takes no explination and is easily digestable to whoever is reading it. It's the same idea as "sampling" in music. Take something everyone's heard, that conveys a feeling and reuse it to bring about that feeling again, but with a new layer added in...hopefully.
I like taking mythology and digging through it. Let's take the vampire example. There are dozens of origins for the vampire they all have simillar characteristics, but still vary greatly from one region to the next. Just off wikipedia, the Greek origin: "The Greeks traditionally believed that a person could become a vrykolakas after death due to a sacrilegious way of life, anexcommunication, a burial in unconsecrated ground, or eating the meat of a sheep which had been wounded by a wolf or a werewolf. Some believed that a werewolf itself could become a powerful vampire after being killed, and would retain the wolf-like fangs, hairy palms, and glowing eyes it formerly possessed." - not the usual origin we hear in the USA!
It's not so much that I pick the most interesting origin and go with it. I prefer to take a oddly logical approach to it. Assuming a vampire is real, why would there be so many different stories of their origins? Are they different subspecies? Is it a series of different curses that result in the same monster? Is it the same origin, but at different stages (as I did with Uhler)...who knows...that's the fun part. I like to take what's available and come up with the most realistic idea I can with what I have.
The most recent one, and something I'm currently writing, is the legend of the Chupacabra. Don't worry, its not a spoiler. If you follow me on Pinterest, you already know this is coming. It's not so much the origin that I'm concerned with, but how the heck it manages to be seen and yet not been seen. In this world of science, magic, mythology and craziness I've built, how can I make sense of this creature so that it works in the stories, is compelling and somehow believable enough that the reader might go "You know...that is a possibility." Something bizarre that can be made believable is what true thrills and horror is all about. I'm not looking to scare anyone, but if I can get someone to 2nd guess their belief in an obscure idea like the Chupacabra and the rest of you to keep reading, then I've done my job. It's all about you suckers....I mean readers :P
By the way, the octopus tree in book 3....that was all me and it'll be a much more awesome idea when I build into it. For now, it's just something M has seen and...well...read Hitori when it's published and you'll have a glimpse of what's to come.