One particularly interesting topic you could analyze has to do with the book's title and which metamorphosis it refers to. Gregor actually has two metamorphoses: a mental and emotional metamorphosis, changing from the confident young soldier in the photograph to the nervous and sad-sack worker who essentially only lives to work and pay off his parents' debts; and the physical metamorphosis, where he changes into a giant insect, that begins the text. Which is really more important? I would argue that the first metamorphosis is the most significant one because it affects Gregor's mental and physical health. It seems like he only turns into an expendable bug after he becomes a worker that can be easily replaced.
Or, perhaps the metamorphosis of the title actually refers to Grete's change, or the change in Gregor's parents. Grete goes from being a sort of gentle and kind girl to one who must harden as she becomes a worker and must help to support her family. Likewise, Gregor's parents change a lot too: they begin in a sort of bourgeoisie position—Gregor pays for their newspapers, lavish breakfasts, servants, and apartment. Gregor's father worked all his life and had little but debt to show for it, and he had become unkempt and rather lazy. However, once the Samsas must support themselves, they become workers as well, taking in boarders and living to serve them. To which metamorphosis does the title refer? Or, might it refer to all of them? This would be an interesting topic for an analytical essay.