I would focus on the American Dream as it relates to new money and old money. In other words, does the American Dream seem possible? Can it really be achieved, since you basically have to have family money? You can't really pick yourself up from your bootstraps.
There are several terrific scholarly essays available at eNotes. Here is a link focusing on the character of Gatsby in particular...
One thing I've always thought was an interesting and rich part of the book is the idea that Gatsby got exactly what he wanted. It may not have all turned out as he imagined, but he is unique in the book in that he set out to achieve certain things, and he did that. A good essay might be one that compares Gatsby's ambition and success with the ambitions and lack of success of other characters. Good luck!
How about focusing on a particular scene involving Gatsby and how it shows his true nature. The most famous is the shirt scene, but that's been done too many times maybe. Maybe a good one to do would be in Chapter 8 where Gatsby tells Nick about when he first met Daisy. It's pretty revealing.