EDIT: Ayumi Tachibana was also discussed in this interview. Sakurai confirmed that she was considered for Melee. Here is the timestamp.
Sakurai made a surprise appearance on a Nico Nico show that talks about video game music and sound effects. The episode centers around Smash Brothers, and Sakurai crashes the party around the 1 hour 15 minute mark. In it, they discuss the music in Smash Brothers, and composers of the game. The guy on the left is Kenji Ito, who composed some songs for Brawl and Sm4sh (Space Armada and a couple of other pieces). Sakurai mentions how the composers can put unique spins on music tracks. For example, Masato Kouda, who worked on the Monster Hunter series made the Vs. Ridley track.
Afterwards, they discuss character selections. Sakurai notes some absurd requests for characters such as Spongebob or Goku. When Tomokazu Sugita (the guy in the middle. He’s the voice actor of Chrom and Takamaru) starts mentioning other absurd characters (Like Segeta Sanshiro), Sakurai says, “There’s always Mii Fighters”.
Then Tomokazu Sugita makes a comment about how happy he was to see Takamaru. Sakurai reveals that Takamaru was briefly considered for a playable character. However, since his popularity was too low, Sakurai felt it was impossible to add him.
In the final part of the video, Sakurai states that Mewtwo is almost finished. He then states that Mewtwo will be released very soon.
Sakurai: But, you know I get requests like, “Please put Goku in”
Sakurai: “Please put Spongebob”
Sakurai: That’s impossible.
Sugita: Spongebob’s games are only sold abroad!
Sakurai: That’s right
Sugita: The really energetic, yellow square right?
Sugita: Even Spongebob…
Sakurai: Well, those kinds of requests are pretty common.
Sugita: What about Yukawasen*, or Segata Sanshiro?
Sugita: …well I’m glad Sonic’s in.
Sakurai: That’s the point of Mii Fighters
*I can’t understand this name for the life of me. The direct link is here.
Sugita: I was really happy to play Takamaru.
Sakurai: I thought about having Takamaru as a player character for a second…but it’s was probably impossible.
Sakurai: Due to lack of popularity….
Sugita: Popularity, huh….I made an appearance! I made an appearance! For popularity abroad…: (In Takamaru’s voice) I won’t give up.
Sakurai has mentioned Takamaru waaaaay back after Smash 64. This is what he said about him then:
Which has a higher possibility…The Mysterious Murasame Castle getting another entry in it’s series, or Takamaru joining the fight in the next Smash Brothers?
Both are extremely unlikely (dang).
Source (Y.H email)