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BKupa666
The main things holding the theory of looking outside of the Smash fanbase is the choices themselves being rather throw out there for Super Smash Bros. fans, but for a more casual audience or pandering for a different fanbase it's not as random. For example, Greninja's and Rosalina's inclusion clearly wasn't due to demand from the Super Smash Bros. fans, though keep in mind that both characters are popular in their respective fanbases. Greninja in particular doesn't even have the excuse of shoehorning the character due starring in the latest Pokemon movie like Lucario, at best all Greninja has is being the Pokemon for a minor side character. Also note how a large chunk of the newcomers cater to the casual market, with Wii Fit Trainer from the ever popular Wii Fit,
Mii who appears in many titles, and Villager coming from Animal Crossing. In fact, I'd argue that only 4 of the characters we know about (including the Sal leak) actually do try and cater to the Super Smash Bros. fanbase, which are Mega Man, Little Mac, Palutena, and Shulk.
Though I suppose considering the majority of what the Super Smash Bros. fanbase wants themselves is already in the game, it would make sense to try and cater to a different audience this time around. This is why the majority of fans for Super Smash Bros. would of be satisfied with only 6 specific newcomers this time around (Little Mac, Mega Man, King K. Rool, Ridley, Mewtwo, Palutena) and would prefer minimal to no cuts.