Describe

In the dark ages, there were castles. Castles had rooms and such that had specific names. If I was writing I just could not say Jonathan walked down from the long ramp to the trading area. I would have to look up castles online or in a book and see what they called these areas. So it would be Jonathan walked down from the bulwark to the bazaar.
It is better for you to know more and make your character more realistic than just saying he would a green shirt. You could say he wore a pale green tunic that match his pallid face as his bright green eyes sharpen and shown even more. The brown breeches (pants) with a tear at the bottom revealing his leather brown shoes.
Everything is determine on the description of a story. I would not say the dew began to freeze on the grass. I would say the *hoar-frost* crunch underneath the travelers foot.
Everything is better when you use a more unique and proper describing word.
You can even describing the scenery, the emotions of two people, or even the senses. We can never cut out describing something.
i love looking up new words and learning what they mean and try to remember most of them. It makes your story want to be read and look up new words at that. A story without description would only be a plot, do not leave anything out that you think you must describe.

Realism

If you want to write about something, you have to first be realistic. i could not talk about how a Minotaur could take down a fort. A minotaur is if a bull could walk as a human. I would have to be realistic and the mix of imagination and realism is how a book is made. A book would be utter chaos if the whole time it was fighting and rampages. It has to mix with a story and what you created.
I first wrote a hundred-thirty-two page story when I was in seventh grade and it was utter chaos. I just wrote what I believe was amazing that blew my mind. I realize in high school from writing different stories that I needed to expand from fighting and focus more on the story. If I can write a story anyone can.
People can do anything they want if they put their minds to it. A good way to make a plot expand is to make a bulletin chapter log. Just talk about the vague details of the chapter in bulletin form and what happens. Then after awhile you will have the beginning, middle, and end. Everything in writing can be broken down into a web or log.
We over think ideas as I fall under this too. I want to make the chapters more insane that it will pop out of the page but it could confuse people and be even harder to explain. Like saying The hero fell to the floor dodging his enemy's attack, he dodge once more then another and he reached for his fallen sword and sliced his enemy down. I could of just said and been more precise, The hero dodges the enemy's attack with a riposte to a stab to the gut. it may not be better but the reader could have a easier way to understand what is going on.
If something is made up it has to be balanced equally with a realistic traits.

Species

What could stop you from making a new species of mammal. You could have a creature with the face of a lion, ears of a bat, and a shape of the head as a mouse. you could make anything reality. Do not think that everything is limited or what your thinking is too extreme. If you want to believe a new type of creature is good in your story or imagination then do it.
Anything can be created in your mind, and I believe what you think in your mind will stick better to someone who wants to read sci-fi. What are you going to remember more a vampire who emotionally depress or a weird lion-bat-mouse combo creature?
Though sometimes people do not change enough, they give a same plot but different names. I believe what ever comes to your mind first you stick with it.
If you want a rabbit who has the abilty to breath fire then do it. I think i would be more scared of that then a real dragon, atleast I can hide from a dragon but the bunny is smaller than me.
If you are going to make a new species do not do what i just did make up a character in three seconds. If you are going to make it research it. Every writer had to research something to make sure what they are describing or telling is what it would look like.
You have to imagine, but you have to be realistic.