Worldbuilding and Continuity. Is your story set in an existing world, or a one completely of your own imagining?
If you are writing something you plan on sharing with the world, and it is in an existing construct, you had better know how things work. Not only the way things work, but what you, as a writer, are allowed to do. Some series that allow or encourage writers to submit new material for their collections have very strict rules. These may include not introducing characters that last beyond your contribution, such as love interests to high profile characters.
If you are building your own Fantasy universe, you have some work ahead of you. I would first suggest reading ‘The Rivan Codex’ by David Eddings. It has step by step instructions that might be overlooked if you are not careful. Plus, I’ve heard he is an OK author.
Make your world worth exploring. Have layers of strangeness that can drive your story, or at least amuse your readers in the breaths between your plot advances. In my current series, the strangeness that drives part of the plot is the way that magic works normally, and the exception that the main character discovers. It becomes familiar to him, and it winds up taking a back seat to other things that are happening in his life. But it is still there, and returns to upend his life once more in the second book.
Keep an open mind. Sometimes the rules need to change to make things work right. Write the rules down. Double, triple check everything that the changes could affect. ‘Apprentice Swordceror’ began as an experiment with rune-based magic, the plan was to have an elder Mage who had been searching for a legendary artifact that granted invulnerability and immortality find new information and lead an apprentice on the quest to recover it. Toward the end of the planned short story, my intent was to have the Elder find the weapon he sought, and discover that it was enchanted with a spell that turned flesh to stone, making the target somewhat invulnerable, and somewhat immortal. Of course, it would have already started, and would be too late. After happening to see a Twilight Zone or Outer Limits with pretty much the same ending, that got scrapped. Once I was about ten or twelve chapters in, a friend who was reading it pointed out a good place for a dream scene. It was just a kind of random comment, but I ran with it. In working up the dream sequence, I wound up foreshadowing the rest of the series, and adding depth to the underlying world mechanics I had only just started mapping out. Luckily, the other notes were all organized, and there were no conflicts that could not be worked out with the new direction.
Finally, immerse yourself in your world. Imagine how the different rules would affect even the most mundane events or people. For example, a society that has magic might stagnate technologically, even though it has thousands of years of history behind it. Why invent a machine if a Mage can cast a spell? Maybe Mages keep the inventors down because they want to keep the power for themselves. Perhaps there is an underground subculture that is advanced, in opposition to the magic users. That simple logical progression could be the basis for a Fantasy world, and provide more than sufficient drive for several plots. It just needs fleshed out and polished.
I should write that down.