Design and implementation
In good design fashion, we need to break down the reasons for the mechanics and interactions that we are going to use. In general, you want to minimize the number of mechanics in a game while spreading their use to many unique interactions. Mega Man is a great example of minimal mechanics with elegant use for slight variations. Locomotion, jumping, and shooting are the only things you have to worry about. After defeating the enemies, you gain different shooting abilities or skills, but you will still use the same button to engage the shooting mechanic. This mechanic is kept to a single button press all the way up until Mega Man 4; when the character is able to charge up his weapon and the button designation changes to adapt to the skill change.
This is an interesting thought: the gameplay involves a very limited number of changes to the mechanics, instead just changing graphics and narrative. When you begin designing this portion of your game, think...