And for the record, if Birdo somehow made it in, I'm damned near sure she would also have an Egg as a symbol.
Perhaps the association Birdo has to Yoshi, and the association Waluigi has to Wario, enough warrants the symbol change.
On a surface level even, the Egg and the W both well represent the characters at hand.
We are also talking about multiple series that have characters of the same universe, who are often found intermingling. Hell, the Mushroom symbol technically means two separate categorizations of series; mainline, and 'other,' wherein 'other' constitutes several completely separate series, given completely separate sales charts, of completely separate genres (Kart, Party, Sports, RPG, etc). I think there is no real way to clearly 'categorize' many of the aspects of Smash into a perfect singular function, and often the lines end up getting blurred because it is simpler not to get overly organized.
I can also just pull up the argument against ATs, and say, "It's never been disproven to not be possible, therefore, it is true."
The real question to me would be- if Ryu Hayabusa got in, would he have changing symbols for DoA and NG, which change based on outfits?
I propose- yes.
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Above is proposed a serious conversation. What exactly does a symbol constitute? For many characters, it is a clearcut answer. Yet for others, not so much. Mario characters are listed on the website as representing Super Mario games- which aren't even the games with Mario that sell the best- Kart games are best selling. It also is a symbol used for other Mario series. Yet, Yoshi's given a completely different symbol. Wario is given a symbol that represents his second solo series, instead of his first, which initially was- similarly to Yoshi- just a sequel to a Mario game (Yoshi's Island and Warioland were both intitially subtitles to Mario games). Is Mario just a spinoff of Donkey Kong? Smash clearly states that they view Mario as having been created at the time of the Donkey Kong games in 1981. Donkey Kong itself isn't 'repping,' those original games, but instead country. But really- isn't he representing both? Certainly there are many people who know the arcade games, and not the Country games; the arcade games made more of a legacy mark in terms of video game history, and even Nintendo history. Are any of the Mario characters even truly from just a single series? Was Yoshi just given an egg symbol in the original game so that the starting 8 could all have different symbols?