Bayo and Ken were pretty good though ngl.
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This being the "Ultimate" Smash Bros, and having the DLC fighters come out so methodically, I do think there is some merit to the last fighter being "special" in some way, but I think it could play out in a few ways. First of all, I don't think characters like Sora or Master Chief even qualify for the kind of send off Ultimate should see. To truly make it a special fighter, the final fighter should come from the Smash series itself, not from some 3rd party fighter, as they aren't even special at this point, not even Microsoft owned characters are special to Smash anymore. So I propose the last fighter should be the likes of Primid (from Subspace Emissary) or even Sandbag; characters made specifically for the Smash Bros series.
I also think this final fighter should be like Piranha Plant, sold individually as an extra DLC fighter not a part of either fighter pass.
However, while I think I'm on the right path with the final fighter being a celebration of Smash itself, I can see it playing out in a very different way. Picture this: after the final fighter of the second pass has been revealed, Sakurai would reveal that not 1, not 2, but 10 more fighters would be playable in Smash Ultimate and soon at that. Naturally we'd be boggled by the announcement, but it would become clear soon what exactly it is he's done. To celebrate Ultimate, after the second fighter pass, they'll add a brand new mode: Boss VS, where one player chooses one of the 10 in-game bosses (Giga Bowser, Master Hand, Crazy Hand, Galleom, Marx, Ganon, Dracula, Rathalos, Dharkon, and Galeem (with Master Hand/Crazy Hand and Dharkon/Galeem being optional as 2 players controlling the bosses at the same time)) and fights as them against all other players. Hell, one of the stages in World of Light actually let us control Master Hand, they've already teased such an idea. Seems like such a Boss VS mode could be an online mode too.