Hey guys, at this point we're kind of just going around in circles with this discussion.
I think the general consensus is that having an offline local multiplayer mode would be a great addition to the game. Having said that, I'm going to assume that Nintendo did consider adding an offline local 4-player mode since their other games have it (IE. Smash & Mario Kart). For whatever reason, it did not make it to the end product. I'm sure you're aware that the entire process of creating a game and selling it involves more than just game design (which has been mostly what this thread has been about so far). While this thread has been focussing more on "they could do this to make it work", let's consider what COULD have prevented them from adding this aspect to the game. Perhaps on the business side of things there were time and budget constraints. Perhaps on the developer side of things, there were hardware and design limitations. Maybe they could not design the mode in a way that would continue to meet the quality of all other aspects of the game. Heck, they could have been lazy. All of the mentioned could have played a role in being unable to prioritize or carry forth adding this mode to the game. I'm sorry that my post has been a little vague and general, but it is another perspective on things.
At the end of the day, Nintendo is a business. They are a for-profit organization and money is what matters. Keeping customers happy is one performance metric to increase profits and improve branding. I think BronzeGreekGod and others in this thread has some great ideas on how local multiplayer can be designed and implemented. If you really want change, voice your thoughts, opinions, and ideas directly to Nintendo. Send a letter or email and let them hear your voice that you're unsatisfied with this aspect in the game and is ultimately preventing you from purchasing. I don't mean customer support either, look higher on the chain. Perhaps public relations or marketing is a good start (they will get it to the right people). There is a saying in customer service that goes "for every voiced complaint, there are X additional customer who feel the same way but did not say it". Gather and formulate your thoughts, opinions, and ideas constructively. Speak and they will listen!
INB4: Nintendo doesn't care what you think! (they will if they feel there is $$$ to be made)
TL;DR
-Offline 4-player would be great
-Process of creating and selling a game is more than game design
-Devs are not the only ones to blame for problems and lack of content
-Let Nintendo know why you're unsatisfied and MAYBE things may change in the future