Plagues and other great diseases are important parts of history and some settings. They defy man and may even defy God in their swift and terrible punishment upon the body of society.
My issue with modifying/removing dwarves and elves is that the PC’s may want to play them.
One of the most powerful tools found in the World Building 102b article is the Worksheet. Here is a place to put the settings you might make with them.
Magical Fiends: When a mage or wizard casts a spell and draws more energy than s/he needs Magical Fiends are drawn to that spot and they feed of the extra magic as long as they can. When someone's magic is being fed off of they can't cast spells as well and they feel weaker. The only way to remove a magicial fiend is for it to be drawn away by someone else's power or to go to the very spot you first cast the spell and kill them