I'd have similar rules to those of KumaOso if I could choose the rules, but I'd still have some differences. I think they should be something along the lines of this:
1. Obviously popularity is one of the greatest factors. If there are lots of people that have been hoping this character would get in since before SSBB was released, then said character should be considered. Also characters that have been gaining popularity recently and have had big roles in recent games get a big bonus in this category. But old characters with big followings like Geno are not out, since they still have enough popularity. And some characters can still be considered even with low popularity if they would be a truly great addition. (Vague I know, but use common sense.)
2. The character should have a new and non-lame moveset. Characters like Toad have been wanted for a long time, even before melee was released, but can anyone come up with a moveset for him that won't just make him seem like a generic character? If not, then his chances are slim.
3. If the character is not the main character from his/her series, and the main character is not included, or some other, similar, yet obviously better character isn't going to make the cut, then said character will not make it in. E.g., Olimar should not be replaced with Louie with removing Olimar completely. And for the second part, an example would be that Mewtwo is better than Alakazam, so if Mewtwo doesn't make it, then neither will Alakazam, since Mewtwo is so similar yet obviously better.
4. Honestly I'd say a character had better have been in at least 2 games to make it in, but I must admit this is not a guarantee, as some characters that have only been in 1 game, such a Geno, still have a chance. But a character that's only in one game better have at least as much of a role in it as Geno did in his one game.
5. A third party series should not have more than one rep without a very good reason. E.g. I vote for no knuckles, tails, shadow, etc. And any third party character that has no ties with Nintendo better have something insanely epic about him/her to be considered.
6. A character must have something that will make at least some players feel excited to play as that character, or a feature that just feels epic. This can be pretty much anything and almost any character with enough popularity and that satisfies number 2 should easily meet this. Even "has an axe" or "can cast earth spells" is acceptable for this.
7. A character should not already be in use by another character in a way such that it'd be very bad for the other character to lose the new character. E.g. sorry Waddle Dee, Waddle Doo, Gordo, Nana, etc., but they're not going to be added as new characters on their own. Peach's B could be replaced with Toadette or Toadsworth I guess, but it seems Toad has a lot of strikes against him, but he's still not out.
8. The character should not be hated in Japan or he/she is pretty much out.
9. A character that's playable in ways such as grabbing a metal box, mushroom, smash ball, or other item should not be playable. E.g. Metal Mario, Giant DK, and Super Sonic are out.
10. Any character that has been in 20 or more games non-third party games and has a reasonable amount of popularity automatically can be considered.
But, in the end, it'll just be people try to bring up characters they want, and if said character is not worthy, not enough people will care to discuss the character so he/she will fail that way; these guidelines are unlikely to be followed.
