I think that your proposed modifications are headed in the right direction! I do have a few questions:
- What do you think the layout of your third level will be? How will it be unique from the other levels - what challenges, obstacles, and environmental aspects of gameplay will you use?
- How will you change the scenery? Are you planning on creating your own, custom assets?
- Will these harder challenges be a part of the overall gameplay, or just the third level? I'd be careful with putting everything in one level - sometimes challenges are a part of a level and may not necessarily be too different of a modification.