But popularity has always been a guiding theme for picks. I guess what I'm trying to say is that popularity doesn't matter any differently than it has before, because it has always been a relevant factor for Sakurai when determining the roster. I don't think this is any different with Smash Ultimate than it has been for the past games.
Sakurai created a Japanese poll for Melee newcomers, and we got most of them: Bowser and Peach were 1 and 2, Ganondorf was 5th, Mewtwo was 6th and Marth was 11th. Wario placed third and was almost added instead of Dr. Mario but Sakurai thought he deserved better. However, of course, we got him last game. Also joined by him are Dedede (4th place), Diddy (16th) and Meta Knight (19th). Ash from Pokemon was also in the top 20 and may have inspired the creation of Pokemon Trainer next game as well.
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We had a similar poll for Brawl, which got us, unsurprisingly, multiple popular picks as newcomers. In addition to the aforementioned Dedede, Diddy and Meta Knight, we also got Ike, Olimar, Toon Link and Lucas. Mega Man and Villager also scored well here, and, as we all know, got picked up in the next game. Sonic didn't get mentioned here, but I don't think his popularity pre-Brawl can be measured on any known scale. Ridley and K. Rool's popularity starts here as well, and they got added recently, and we know Sakurai was always aware of Ridley's popularity. But I don't need to debate that point as we know popularity is playing a factor here.
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We don't have a poll for Smash 4, a first, and so far unique, situation for Smash. Anyone who was here in 2014 can tell you that we got quite a few popular characters: the aforementioned Villager and Mega Man as well as Little Mac, Ryu, Cloud, Shulk, Mewtwo's return, Roy's return, and Rosalina (a bone for casual Nintendo) all demonstrate this, as well as lesser, but still popular, characters like Robin, Palutena, Duck Hunt, Pac Man, Bayonetta and Lucina. I think we stopped thinking popularity mattered when we saw all of Dr. Mario, Wii Fit Trainer, Corrin and Dark Pit (and, due to unique circumstances, debatably Greninja) on the CSS as no one wanted them, but every game has had a few odd-ball picks (Game and Watch and Ice Climbers, Pichu, Roy and Dr. Mario in Melee, and Zero Suit Samus, Pit and ROB all were those odd picks that Sakurai loves to throw in).
What a lot of people (myself included) have been viewing popularity has the sole guiding force in determining the roster due to the constant stream of "highly requested" picks we've gotten, when in reality, this situation with Ultimate isn't any different than in the past.