I think that your proposed modifications are headed in the right direction! I do have a couple questions:
- How will your levels be unique and significantly different from each other? What types of obstacles and challenges will you use?
- When you say additional graphics, what does that mean? Are you thinking of sculpting 3D models, or altering the 2D user interface / backgrounds?
- How will the enemy behavior be different? Are you planning on writing scripts, or modifying existing ones?