What two arguments could I talk about in the theme of individuality vs. nature in "To Build a Fire"?
Your question certainly recognises the central conflict in this great short story of survival and death. Man is pitted against nature, and throughout the tale, nature is shown to be stronger and more powerful compared to the insignificance of man. However, if you are after evidence to prove this, I would look at the text and consider the way that nature is presented, and how, even from the beginning of the story, the sad end of the man is foreshadowed:
It was a clear day, and yet there seemed an intangible pall over the face of things, a subtle gloom that made the day dark, and that was due to the absence of sun. This fact did not worry the man. He was used to the lack of sun. It has been days since he had seen the sun, and he knew that a few more days must pass before that cheerful orb, due south, would just peep above the skyline and dip immediately from view.
Consider how this quote, through its reference to the "intangible pall" and "subtle gloom" that darkens the "clear day," suggests that something bad is going to happen. Yet, at the same time, as happens again and again in this story, "the man" (who interestingly remains nameless, thus reinforcing the insignificance of man against nature) is blind to the warning implicit in this scene. Throughout the tale he is shown to be overconfident and arrogant, which leads to his death. So if I were you, I would explore these themes and see how they are presented throughout the tale. It will give you more than enough material for an essay. Good luck!