I think that you could write about how the author came up with the idea. Or how it affected the readers myabe...
Compare the moral and ethical dilemmas that Victor Frankenstein (should have) faced to the same dilemmas stem cell research currently faces. Did the role of science change or do we still need the stereotypical "mad scientist" to progress?
There are so many themes to base an essay on, but another good one might be to write about the theme in the story about man attempting to play 'God' and attempt to mess with nature and elements that he does not fully understand. Then relate that to mankind's continued attempts to change and dominate nature (hybridized plants for example). There is also a huge feminist theme running through the book, too.