Some great advice up there!! Pay particular attention to the avoid bit, I pretty much agree with them all.
Also hello again sweetalmond! What do you actually need help with? Can you be a bit more specific?
Regardless, here are a few things off the top of my head.
Incorporate the source well in the HSC is half the job done
The creative piece you wrote utilised the requirement of a letter being present in the story well - at least in the eyes of the markers anyway. I've read many great prepared creative pieces last year but it doesn't mean anything if you can combine the story with the requirement on the day. So just keep this in mind throughout the year. Open your story up, don't make it narrow and thus difficult for yourself.
It really doesn't need a lot of "action" and happens over a long time
The best creatives and short stories I have read are the ones that get right to it. Be realistic. You only have say, around one hour to execute this creative piece. If you want to carry this story over a long period span (say a few months or years whatever), chances are you will lose a lot of time in 'story telling' and not developing the story or your characters. For example, my story happened over one night, with around 300 words setting the background and tone of the story in the beginning. You want to win in detail, broadness doesn't really help in english extension. It's the same for essays. Sure, nature is big in Romanticism but which aspects of nature?
This brings me to the second point, because of the focus of the short time span (as recommended), having a lot of things happening at once is not ideal. Have that one or two things you story focuses on and develop it well! Otherwise, in ~1000 words, you will just be wasting time and confusing your readers. It's a short story, not a novel.
As Erique said, do start off with a character profile
If you don't have time now, forget it, but eventually it'll help you a lot by creating your 'ideal character'. Do the background (thus influence), personality, aspirations etc. This will help you in your story to come up with their response to situations and really underpin the character development and that is crucial for engaging audiences and help them relate to your story in one way or the other.
Research about Romanticism
My school did creative last, which I really appreciated. I mean, doing it so early in the course, chances are your romanticism knowledge isn't that strong yet. There were probably a lot more subtle romanticism ideas in there that you couldn't spot. So, do those extra research! Romanticism is one wide ranging movement and it included SO MANY, SO MANY IDEAS (I mean it did happen over a bloody long time) so go more into do it and on the way you should find your inspiration and that's what happened to me.
eg. the preface of Lyrical Ballads and The Grasmere Journal of Dorothy Wordsworth. Not only do they serve well to extend your knowledge and maybe you liked it so much they became your related text as well - again, what happened to me.
Incorporate real events of the time to support your story
I'll be honest, I started off really cliche stories at first and most ideas were really on-the-surface stuff and it was clear to the marker that I didn't have real deep understanding of the historical event that is romanticism. Eventually I was told by my teachers that HSC markers love it when you can show historical understanding to help further develop your creative piece. So, as the point above suggested, I researched actual happenings of the time in Britain, France whatever.
In the end I found the Enclosure Movement which I was fascinated by, and so it became the event that became the catalyst of the happenings in my story and what triggered my character's actions and response. Of course, do make the link to Romanticism. In this case the Enclosure saw farmers forced to be removed and the wealthy took their land - baby steps to industrialisation really - as they were deemed to be what was keeping society back with their lack of intellect and rural ways. EX-ROMANTICISM STUDENTS SHOULD SEE SO MANY LINKS HERE ALREADY.