Obviously these ideas won't apply to all stories as every story is different.

SciFi
-simple design which represents book
-clear, bold writing
-bright or dark colours
- interesting

Horror
- dark theme (greys, blacks, reds)
-thought provoking
-creative
-doesn't give too much away

Romance
- tumblr will be your bae
- cute swirly fonts, italic fonts
-softer colours

Mystery/Thriller
- simple
-if there is an object that is key to the plot, have that on a black background with just the title and your name

Fanfiction
- picture of celebrity
-clear writing
-refrain from too much colour

Random
- whatever relates to your book
-clear writing

Humour
- using a funny line from the book as a subtitle might be a nice touch (a short one though)
-simple cover

Non- fiction
- CLEAR AND SIMPLE

Fantasy
- colours, cool pictures
-you can just go crazy on this one tbh