I hate to be "that guy," but as a developer in training, I want to talk for a minute about the business and logistical realities of what really goes on during development. Let's forget the gameplay considerations of whether other stages might be too small to be fun with 6+ players. Let's even forget about Nintendo's unwillingness to make changes to their policies or development practices based on petitions.
The game is set to launch in less than three weeks in North America and Japan. Even if the petition had 10,000 signatures tomorrow morning and Sakurai showed up to work with a copy on his desk - even if he went back on his current word regarding why he chose to restrict the stages - the features and content that are in the final version of the game have been locked in for weeks - if not months - and there isn't enough time for it to happen.
Software development follows a very controlled process of conceptual design, business considerations, systems design, programming, testing and analysis. Even with an agile approach to development (which has a faster iteration time and can make changes like this more easily,) it takes time to go through the entire cycle. I can promise you that the Wii U version dev team is in a serious time crunch right now, and just about everyone on staff is probably working overtime to get the last critical touches done in time to ship the final code off to manufacturing so you can play it on the 21st.