Event matches! Bowser's animations! DK item voting!
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SmashBoards has created: Super Mario Smash Bros!
DK Items Coconut Chill This is a healing item that produces a coconut that you can throw at opponents that freezes Crash Guard This is a reskin of the Back shield

Video games are something with a like half-century long history with a lot to it and a lot of people to please. The only way you could even begin to create a (fighting) game that really qualifies as a celebration of gaming would be to have an absolutely massive roster, created by a new, completely independent development team for the specific purpose of creating such a game (with all the funding and resources available to the biggest name games). That can also meaningfully implement aspects of gaming history into the game that might not translate well into playable characters.
But it's not something that we can expect to happen, it would be a task possibly more enormous than the industry has ever tackled before