The claim that Sonic or Mega Man, characters that received incredibly high positive reception when they were revealed, don't generate interest in a crossover fighting title comes off as asinine and biased. Hype is a huge factor that Nintendo knows Smash fans eat up, so the easiest way to generate hype is fighters. Traditionally, guest fighters are what generate the most interest into any game.
Guest fighters always will gauge more interest in crossover games. I'm pretty sure more heads turned when it was announced Pac-Man was announced to join Smash than when Palutena was. But, Pac-Man probably convinced less people to investing in Smash than say, Cloud or Bayonetta. Those characters were definitely dreams come-true in comparison.
That's not to say a character owned by Nintendo cannot have similar responses. I'm sure many people yearn for a character like Isaac to appear in Smash, and the people that are still hoping to see the likes of King K. Rool or Ridley. But if you're already a Nintendo fan, the likelihood of you already purchasing the game is very high. While someone who has very little interest in the majority of Nintendo's IP outside say, Zelda, would be more interested in seeing characters from games they play more of. Cloud is a pretty good fit for this example I feel, the huge draw-in for Cloud (in my opinion) was the title-bout of Master Sword vs Buster Sword became a reality. It was literally the stuff of fan-fiction, realistically Isaac and most of Nintendo's currently non-playable options don't compare to that for the general player base. It's just pretty common business-sense if you ask me, providing an experience you can't get anywhere else will get people's attention.
Additionally, you can't say the novelty of having Ryu team with Mega Man to fend off Bayonetta and Sonic on Midgar while Waluigi is summoned to assist isn't amusing.
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Comparing sales figures for a Wii title and Wii U title makes no sense. Everyone had a Wii, barely anyone bought a Wii U. Of course it sold worse, nobody is taken aback by that factor. Claiming that this proves that the "selling power of Guest Characters doesn't exist" makes no sense because it works for many other fighters.
Tekken 7 has stayed relevant in terms of sales for quite a long time, and I feel that should be credited to the content they've been offering.
- Akuma from Street Fighter being in the base game had to brought in some SF players to try out Tekken.
- Geese from King of Fighters as DLC #2 had to brought in KoF players to see Geese actually exist in a Arena fighter.
- Noctis from Final Fantasy XV literally brings people from a different genre.
You know a game that didn't sell well? Marvel vs Capcom Infinite.
- Poor presentation.
- Spotty Roster.
- Really good gameplay.
I feel like the general agreement is that if the game had characters people wanted to play as, it wouldn't be in as big of a pickle it's in now. People will play a game if it has their interest, not many people were interested in playing as Gamora over Wolverine.
My point is that characters sell. This is true for absolutely everything regarding fictional media, whether it relates to film, television, books, or video games. If people care about a character they'll buy stuff with them in it.