64 roster:
Additions: Charizard, Banjo
Cuts: Jigglypuff, Ness
[collapse=Reasons]Charizard/Jiggs: Much more popular and cooler than Jiggs.
Banjo/Ness: I'm not really familiar with Ness outside of Smash. Not to mention his game got only released in Japan. Someone else had to join, and I had no idea who that would be. Banjo also was really popular at that time I think.
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Melee roster:
Additions: (Charizard), Diddy, Scizor, Ridley, Dedede
Cuts: (Jigglypuff), Dr Mario, Roy, Pichu, Young Link, Sheik, Mr GaW
[collapse=Reasons]Banjo got out, because of Rare-stuff (I also forgot to include him, I think Rare left later, but w/e).
Diddy included, because he's the 2nd most important character in DK games, and DK games were really popular.
Scizor instead of Pichu: 2nd gen rep that isn't a troll like Pichu was.
Ridley: pretty popular
Dedede: Kirby villain
Dr. Mario: no need for 2 Marios.
Roy: I don't feel like FE was big enough around the world at that time... but it's probably just me (adding 2 FE characters might be a bit much anyway...)
Young Link: no need for 2 Links.
Sheik: Zelda is enough, for now...
Mr GaW: dunno, but we already have ICs added as a "retro" character.
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Brawl roster:
Additions: Bowser Jr., King K. Rool, Midna, (Mewtwo), (Scizor), Deoxys, (Ridley), Samurai Goroh
Cuts: (Jigglypuff), (Mr GaW), Toon Link, Pokemon Trainer, ROB, Lucas
[collapse=Reasons]Bowser Jr.: Mario deserves another rep (even though Wario kinda is one already, but he has his own symbol...).
LoZs new reps are Midna and Sheik.
K. Rool: pretty popular and DK deserves a new character...
Deoxys: 3rd gen rep, also had a main roll in a Pokemon movie.
Samurai Goroh: 2nd rep for F-Zero.
Toon Link: no need for 2 Links.
Pokemon Trainer: no need for such a gimmicky character with not as popular Pokemon. Charizard is already there, and Deoxys and Lucario are the new ones.
ROB: I don't like adding random troll-characters.
Lucas: I guess I rather added other characters than him... even though Mother 3 came out, it doesn't seem like there's a true need for him... I guess he could've replaced Ness, but meh...
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Smash for Wii U roster:
Additions: ??? (we don't know all characters yet)
Cuts: Wii Fit Trainer
[collapse=Reasons]Wii Fit Trainer: see ROB.[/collapse]