Another old idea of mine.
Cassiopeia is snake based and uses poison attacks. It shouldnt be that hard to change it to fire for a skin.
The concept was a pretty rough first visualization.
Later I decided to make another splash art and put more effort and thought into the design and flair.

And the link to the LoL Forum for this idea:
http://boards.na.leagueoflegends.com/en/c/skin-champion-concepts/BZozAb1O-skin-idea-dragon-princess-cassiopeia