I went to a panel about good art direction for comics/screenplays/etc. It turned out to mostly be a panel about being art-directed by an art director, but I did manage to pull out some tidbits of advice for figuring out art direction on your own.
First off, you’re trying to tell a story through pictures. What’s it about? Make sure the art focuses on that.
Be creative. Figuring out the art style is the first step after the story is written. It is affected by final format, meaning TV vs. Movie screen, paper size, or unlimited internet canvas.
Research. Pull from nature. Pull from other cultures. Find things you can reference and flirt with in your style.
Don’t forget all the props! There will be a ton of them.
That was about it. I hope that was helpful in some way.