well, my topic is "Representations of Adolescent Gender and Sexuality in Buffy, the Vampire Slayer", which developed from my original concept of examining aspects of morality in BtVS (which I eventually changed because I realised it was much too broad).
With the critical response, you can really write about anything that you have a sophisticated understanding - but you must be able to tie it back to the Prelim or HSC Advanced/Ext1 course somehow. This is actually quite easy - the way I did it was discussing my study of Frankenstein in the Prelim extension course, and how I became intrigued with the ambigious notions of morality it contained, especially as I used Buffy as a related text for my Frankenstein MRP.
Does this help? It's more showing by example rather than saying "this is the universal answer, yo". Sorry. : )