2D Shooter

Your proposed modifications sound okay, just be sure that if you are reskinning without making your own sprites for at least one part that you reskin the entire game with sprites that match to your chosen theme (each individual sprite you make yourself can count as a significant modification just be sure to put your best effort into the sprite). New details on levels would fall underneath modifying or making new unique levels which can be significant modifications so long as each level that would count is unique from the other levels you created. The new characters would certainly count for significant modifications.