So I have a question for you all, which newcomers do you think are essential for a roster?
I read the responses to your quote, it is quite laughable that most are laboring under the delusion that we have all of the essential characters. I suppose it is a very subjective notion. However, I would assert that we are missing a few blatantly necessary characters.
1. Meowth: The main antagonist of Pokemon. Obviously one of the most prominent franchises should include the spiritual rival of Pikachu (inverted Pokedex numbers alluding to the premise of cat and mouse) along with his iconic role as the most important villain in the popular anime series, the same anime series that has given Mewtwo global recognition.
2. Black Shadow or Samurai Goroh: As if the overt Smash relevance didn't make them seemingly necessary, F-Zero is the face of Smash when associated with Captain Falcon, one of the most popular characters. So much, that only the main antagonist of Legend of Zelda could possibly satisfy a role essential to Smash. One might argue that the series is not relevant to Nintendo, however, this is obviously fallacious, the series is relevant to Smash. In a Smash related context, nothing is more important.
3. Tom Nook: I don't know a lot about Animal Crossing, I do know that he is the face of the series though, so by default, he is essential. I don't care if Villager is "proper representation" that wasn't your question. You didn't ask who the most likely newcomers were, you asked which newcomers were essential. Animal Crossing isn't accurately portrayed without the most prominent character, does it have to be? Not necessarily. But is it? No.
4. Hades: He is one of the only Nintendo characters that represents more than his character in Kid Icarus, more than the franchise Kid Icarus, and MUCH more than a Nintendo character, MUCH more than Nintendo the company. He is the effigy of evil, global recognition doesn't even begin to satisfy what he represents. He is more important, iconic, and prevalent than even Mario himself. He is what Kid Icarus needs to be a main franchise on the roster, just Palutena won't cut it. That would just acknowledge Kid Icarus Uprising, we need something to make the series eternal. Something to make that IP valuable, something to make it akin to Mario and Zelda. Hades could be that character.