Sounds like you're having the same issues as me, too many awesome possibilities! I think the hardest part of the Major Work will be just choosing one focus area, and then deciding on a question.
Just be careful with Gandhi. He's a key component in the Modern History Year 12 syllabus for the 'Option D: India 1919–1947' in the National Studies Section of the Modern History Course. Basically the content in the course involves evaluating his role, ideas, and impact on Indian Nationalism. So just be weary of what direct path you choose for the Major Work, and once decided double check with your teacher on if he/she has a problem with minimal overlapping. Board of Studies states 'significant overlapping of other examinable HSC and SC History Courses' but I think you would be okay, since Gandhi in Option D is very small, and you would be looking at him much more in depth!
That being said, perhaps you could debate his primary role in India; was he a political figure or a social reformer? Maybe analyse his legacy on India? Perhaps an evaluation of his actions in India, were they significant for people / country? why? Umm, maybe argue his most significant decision / action in his life?
There's questions you could derive from the Modern History Syllabus (and also the Ancient History Syllabus when looking at personalities), there's quite a few Historiographical questions, such as evaluation the role of the personality in their context, analysing the legacy, their impact in the times and the future of the nation in context, impact of personality on history.
Hopefully this helped a bit.
Good luck on your endeavour with the Major Work.