The best stories are often those which are authentic in nature - that is, anecdotal. I'd suggest doing something that you've personally experienced or something you know very well. Put in the effort to develop something amazing so you can bring it on into your HSC.
My story was incredibly cliche (young girl w/ bad dad plays music --> dad tries to be good, turns out being bad in the end, but the girl finds strength in the good moments that she experienced w/ him and lives those experiences through a piano), but I received really good marks throughout the year because of how realistic it was (despite me not being a girl and not playing music - I did a BUNCH of research).
I find the simpler stories packed with meaningful utilisation of language that SHOW and DO NOT TELL get the better marks. I was told by a past senior HSC marker that you should try not to be explicit in what you present to markers - you need to trust that they will understand the complexities of your story. That said, endings should never be "in your face", but should be completely up to interpretation.
All said and done (I know I went waaaaaay off topic here), do something you've experienced, something you know well, or something that is close to you.
I went with a coming of age story (that DID NOT link to schoolyard politics in any way), because all markers (despite being way older) know what it feels like to be disappointed, happy, relieved, innocent, etc (or have children that may relate to the same situation). Find a medium that symbolically/metaphorically presents your idea. Yet again, do not be explicit and do not utilise any taboo ideas.
Perhaps you relive a moment of your childhood where you felt an unrivalled intensity that changed your perception of the world - exaggerate it 3 folds and pack it with beautiful imagery and other techniques). I'm also going to assume you're in year 11 too - don't be afraid of making multiple drafts... get these all marked and continue working towards synthesising a story that will get you 15/15.
