2D shooter tutorial

Your proposed modifications seem a little weak overall. The boss fight level will certainly count as a significant modification. The different backgrounds however seems to simplistic unless you are making your own art for this, at which point what you create will absolutely count for the significant modification. Asteroids within the game level would be a part of making a new unique level, on its own it is too simple, but if you create new unique levels each one can count as a significant modification you just need to make sure that each level is not too similar to your other levels.