Hi,
What would you say would be some topics or forms of text that should be avoided or at least tread carefully upon?
Thanks
To give two very specific warnings, speculative fiction and mental illness. Avoid speculative fiction because you can't both set up a deep world and execute a compelling story in the limit of 6000 words, or at the very least it'll be incredibly difficult. Avoid mental illness because the vast majority of people that write on it either misportray it or rely on it as a crutch for a poorly-conceived story, and both will alienate markers.
As for forms, I'd say that nothing is off the table. That isn't to say some forms will serve you better than others, though. It's a bit of a truism, but you'll do best in whatever form you're best in. If you have no experience with drama, a script or screenplay will take much more effort than a short story to produce a work of the same quality. But on the other hand, don't rule anything out just because you think you couldn't do it. HSC markers are paid to be unbiased -- your passion for the concept and form you're doing will affect your mark, but marker preference will not. And teaching yourself how to write in a form you're unfamiliar with would be incredibly rewarding.
I know this isn't what you were asking, and forgive me if this seems contradictory to what I just wrote, but some genres can give you an edge and improve your odds of writing something that will really 'wow' a marker. (Modern) Historical fiction is good for a number of reasons: the realism of the world you write is limited only by your research. History is a 'resolved story' and provides known conclusions for the cultural backdrop of your story, which really helps a reader engage with the world your characters inhabit. History lets you weave your story into a context of social and political developments, for example, Jazz Age New York immediately gives you a set of ideas with which to run such as race, pride and counter-culturalism. By extension, revisiting such a 'resolved story' to appropriate those ideas into a commentary on contemporary issues lets you make some very profound statements. And in the end, that's what an audience loves best -- a piece of fiction that 'says something', and leaves them thinking about it.
Good luck, and if you haven't already decided on doing extension 2, I strongly suggest it