The one fun part about creating a new Breed is developing their character traits. With the Chitah, I thought about permanent markings but decided to only make them visible when their emotions are out of control and their inner animal is fighting to get out. I also wanted to make them stand apart from a Vampire by having lower incisors in addition to upper. It was also important that they have features that make them stand apart from the crowd, so they could be easily identified. Being very tall is one trait, although anyone can be tall, so I gave them unique eye coloring. I took a quick pic of my own eye to try and experiment with the eye coloring. In addition to the color being yellow to an amber/light brown hue, they're usually rimmed in a dark color. Not always - although I may not elaborate on this in the books as it's one of the details I just have in my head. I don't like to ramble on and on about eyes, but a future book or series may touch on some differences.
Book three will introduce a new Breed mentioned once in a previous book but never elaborated on. I'm a little excited because I didn't want to throw a bunch of new concepts at the reader in the first book. I like the slow immersion into the world so the reader can gradually acclimate to the different Breeds. I wasn't sure how readily the Chitah would be embraced because they're not a Shifter, and yet they do have an inner animal that can completely take over in the sense that they run on primal instinct. You can reason with an animal, if they feel threatened they react in a very simple way. Anyhow, I hope that each new Breed that's introduced in the series is interesting and exciting. This one is different from all the rest.
Sensors were fun to write because I liked the idea of a person who could store emotions like a filing cabinet and sell them, as well as be able to pick up residual emotions left behind. They ideally would also make pretty good detectives. Of course most of them find the money is in the trading.
All I can say is that writing isn't just about taking a world and living within it, it's about creating your own. I would strongly encourage anyone writing fiction - specifically in Fantasy - to step outside of the lines and do something different. I can count how many Vampire books I've read and loved, but wished they stood apart from the others. The problem with that is that Vampires are very hard to write different because we have stereotyped them to DEATH. While they appear in my books, there's nothing dramatically different about them. It's too chancy to rewrite what a Vampire is without making them something entirely different. I took the chance with a Mage because they're not often written about, and I could completely see a misinterpretation in how humans saw their power, and where that power really came from.
This is why I love how children write, because I don't think they're bombarded with all of the stereotypes just yet and their imagined characters are often strange and unique. Reinventing the wheel as an adult doesn't come easy, but you only need one character trait to build upon. I think it makes your world fresh and different, and whenever the characters make an appearance, I would hope that the reader sits up a little in their chair because they don't know what to expect.