Choose what you think will be the most interesting and fun to do. I did architecture this year, not sure how I went, but it was really fun and interesting, so I liked it. That's what you want - you want to like what you've chosen for your major because you'll put more effort into it (although now I cringe at the thought of doing another drawing).
In terms of actual ideas, the list is endless (these are some from my class):
- Furniture
- Devices (phones, game consoles...)
- Architecture (a new building or an addition to an existing one)
- Jewellery (such as watches)
If you feel like you could do architecture, I wouldn't reject that idea. It looks impressive because of how big each concept and drawing is, so it looks to the markers like you put a lot of effort and time into it. Also, you can get away with using CAD software more, because in reality, house designs aren't hand drawn too often anyway. And another advantage is that those architecture questions that you'll get in your HSC will seem so natural to you.
Also, one piece of advice: whatever you choose to do, start your major now. Get most of your folio completed by the Christmas Holidays and really start getting your designs done over the holidays. If I could redo this year, that's the one thing I would change. You don't want to be doing your trials but worrying because your major is almost due. I know you'll think you have so much time to do it, but my whole class this year thought that too, and now we all wish we had really gotten into it when we had the chance.
Good luck