I think the hardest part of coming up with an idea, for me, was the whole sense that it had to be good and literary -- that it had to be the sort of thing that English teachers would like, rather than the sort of thing that I enjoy writing. I remember at the beginning I was freaking out, and struggling to come up with something, and getting nowhere.
In the end, I went against my teacher's advice and did a fantasy major work, because that's what I'm into. Once I made that decision, everything seemed so much more fun and less frustrating.
I think the important thing is coming up with an idea that you feel passionate about, or that you think that you'd really enjoy writing. If you do that, you'll be happy to spend the hours it takes to create a good major work. Sure, listen to your teacher, and take in their advice, but also be willing to do what you're interested in doing because it is your major work.
I ended up finding a compromise with my teacher -- while it was still fantasy, she was cool with my concept, and I've ended up with a major work that I'm happy with. Remember, while you're trying to come up with the seed of your major work at the moment, it'll still grow, adapt and change as you write it. I'm probably one of the few who kept the same basic concept from start to end -- but my concept was quite open, and how I wrote it changed significantly throughout the year.
The main advice I have is to stick with what you're interested in -- what you love to write. Come up with an idea that you'd enjoy writing. The quality -- the literariness -- of your major work is something you can work on, but only if you've got something that you're gonna enjoy writing. Don't panic about making it perfect now, or that'll just paralyse you. That's something that can only come with time.