Right now I'm seeing a Luigi's alternate outfit in black and purple for everyone's joy...
No, seriously. If Waluigi had more predominance beyond Mario sports and party games he could have a chance. I'd like to make a quick review on the main used reasons to support Waluigi in Brawl:
1. Waluigi could represent the Mario spinoffs titles, like Party and Sports.
I doubt it. This is because Mario spinoffs titles are considered by Nintendo as Mario games, which are being repped by Mario and his crew as Mario characters.
2. But there is no actual reference to these titles in terms of attacks and stages.
Peach took the "character made from holy scratch", using sport gimmicks and such borrowed moves from diverse Mario titles to complete her moveset (since Peach doesn't has too many fighting references). It would be somewhat stupid to have another sport based character. And no, it isn't the same as having 5 swordmen since there are a lot of swords and styles, but sports don't vary. Mario Sport games are Mario games, which are already represented in many ways.
3. He could be a Wario partner.
A character shouldn't be added for supporting or being an accesory for another character. All supporters in Melee were there by their own merits: Popularity, fit-ability, importance to the franchise they are representing. This applies to most characters, with the exception of some clones (Doctor Mario, Pichu, Young Link and Roy). Waluigi isn't a vital part to his franchise (Mario series) as Metaknight is to Kirby or Zelda is to Zelda (lol). The same with Wario in Melee.
Most Mario secondary characters, like Yoshi or DK are representing their own games. What would Waluigi represent? Wario is going Ware, so he prescinds Waluigi. Waluigi wouldn't fit with the Mario classic characters (Mario, Luigi, Peach, Bowser), and he doesn't own a franchise.
By the same arguments, Birdo has appeared in more games that Waluigi, should she be in Brawl then? I think not, Birdo's appareances doesn't mean that much to the Mario series beyond the sport games, Nintendo's story and, ultimately, story of gaming. The same could be said about Daisy and Toad.
Right now, Waluigi doesn't has the merits to enter the Brawl. He started as a partner for Wario, and he may have more important games in the future as Tingle did.
4. But, Luigi is a filler too. What's so different?
Yes, but Luigi is almost that popular as Mario. Luigi has been in any Mario game, including spinoffs, since Mario Bros (1985). He has appeared in all games that made Mario famous (excluding Mario 64 and Sunshine, the last one being fair because he had his own game by then), and he became a recognisable figure by all gamers who owned a Mario game. Nintendo gave him different attributes and personality, and he became stablished. Waluigi is nothing compared to Luigi in terms of popularity, recognition and charisma.
4. Waluigi has more personality that many characters, like Pit, Ice Climbers and a black-beeping 2D figure.
Sure, but characters aren't chosen in Smash based solely on how good or developed is their personality. We would see all Fire Emblem characters in story in Smash then.
More than personality, is about looks, appealing, popularity, fighting potential and importance to their franchises and Nintendo's story, because this game is a big party celebrating that story, characters, games, creators and fans taking part on it. That's like G&W, IC and Pit are so great, because they are raw nostalgia revived and all retro gamers recognise them. Waluigi also lacks that: Nostalgia and story.
5. Waluigi is more recognisable and popular than most characters suggested in these forums.
Sure, Waluigi is more popular than most characters since he appears in some of the best selling titles nowadays, but then, that doesn't mean much. Birdo, Toads, Bowser Jr... a wide cast of Mario characters that are more popular that Waluigi, yet their chances are very low.
And, don't try to compare the popularity of a legend with Waluigi's, cause the first ones have inherent nostalgia and are unforgettable. Like Pit and any other retro character. He is popular among Nintendo fans, and the game is made for Nintendo fans. So, after waiting for years, they reappear.
Most characters aren't suggested by how cool they are (sometimes), but what mean to the suggesters. Nostalgia is the key here. I almost cried when I saw Pit. Waluigi won't likely achieve that, while retro-obscure characters yes.
6. Waluigi is very popular.
Well, Waluigi is indeed popular, but he hasn't a rabid or strong fanbase. By strong I mean something like "OMG, Waluigi, you are DA MAN! W00t!" than "Meh, I know Waluigi. He appears in every game I buy though". Not to the extent of Wario, and yet, Wario is considered an underdog character. I love Wario, so don't look at me. He is so **** freak, japanese-humoresque like.
I think this cover most points. I'm no hater, only realistic. If Brawl mood keeps the same as Melee (balance, compact roster packed with style and glamour, no minor characters except some clones), it's definite that Waluigi won't appear.