It depends what you want out of your major work I guess, whether you want something you are absolutely proud of and honestly enjoyed working on, or something that is going to ultimately score amazingly. In a perfect world you want a combination of both, but I know I'll be so proud of mine no matter how it scores, the most rewarding thing I have ever done and the only thing that got me through year 12. If you look through the showcase books they are all super original with multiple perspectives and so much stuff crammed into them, but that's not to say people don't do just as well, the purpose of those books is to show off the range of ways you can do language and what people
have done within the course.
Look, as my advanced teacher says, 'don't look for a perfect idea, look for a idea to perfect.' You can write it all out and then pick up the bits you think are overused and think how you can give them a new edge. I know people who wrote out their whole story then thought a script would suit it beter and changed it to that, added letters instead of normal narration, changed their whole thing from first to third person, you can always manipulate it! If you do that you will be right. If you'd like, I can email you my major work to read, just to get an idea? I obviously don't have my marks back yet though.
I think, the most important thing, is to enjoy it and have something you are proud of. That is just me though. I think if you put too much pressure on yourself to get an amazing mark you automatically are hindering yourself.