This is fairly wide and you do have much from which to choose. In the end, I think that you are going to have to select the two characters with which you feel the most comfort from the novel. For example, Gatsby's motivation to win over Daisy is evident. His obsession with trying to win over Daisy for his own is something that motivates him, driving him to ends where some level of questioning might be present for some, but not for him. Exploring how this motivation drives him to the highest of emotional highs and the lowest of lows might be something that could prove to be quite compelling in a paper. There are many other characters whose motivation are intensely present in the novel. One that does not get much in way of recognition, both in the novel and outside of it, would be George Wilson. His motivation to actually be considered a man or to be seen with a modicum of respect by anyone is something that can be seen. He represents the lower class of Americans who are constantly being trampled by those in the position of economic and social power. His wife wishes to be one of those people, so she views him with a dismissive eye, and he is constantly feeling the pressure of being invisible. His killing of Gatsby is a desperate attempt to feel some power in a world where people like him have little, if any, control over their own lives or states of being in the world. The pulling of a trigger is something for an instant that he can experience. It comes as no surprise that his instant of power was actually aimed at the wrong person. George proves to be a character where some level of exploration regarding motivation could make a very interesting writing sample.