My main thought here was mostly because of the influence that, say, Dungeons & Dragons or Magic: the Gathering had on video games. Without D&D, the RPG genre as we know it doesn't exist. Without MtG, trading card games - including the Pokemon TCG - don't exist. They might not be video games themselves, but their mere existence has such a huge impact on video games as a medium, enough so that you could justify including them in a celebration of gaming history in a way that doesn't really apply to most popular characters from anime, manga, comics, movies, etc.
I mean, I'm specifically talking about public domain characters. Stuff like Dracula, gods from various myths and legends, stuff like that. And when I said tie-in media, I meant, like, tie-in media based on video games rather than original characters from licensed games. My thought there is it wouldn't even be something third-party. It'd be, like, something from a piece of tie-in media for a Nintendo game, or... I guess Nester would fall into this category.
I think when it comes to original characters from licensed games, the biggest factor in my eyes would probably be how much that character stands on their own (e.g. Starkiller and Android 21 are out because they rely heavily on the context of other characters from their respective games' source material), how iconic they are, and how big of an impact the game has had on gaming. I think the only real contenders there would be, like, Sora or maybe Darth Revan, and I don't think the latter would ever make it into Smash.
