Poor Waluigi isn't really relevant to the Mario series, though.
Even Daisy and Birdo have appeared in more canon games than him (that is, any at all). Regardless, I feel bad for the guy. When they excluded him from Mario Kart 7, I did get a bit ticked off about that.
Still there's a huge line-up of Mario games that would make it into the SSB series as playable first, like Bowser Jr. (unfortunately) and Toad.
But even with that being said, above all other Mario characters, Waluigi was one amongst only three total to become an assist trophy, and actually be hacked into playable form within the game during a long, thought out process including many hard working fans. If anything, I think Waluigi has one of the highest chances of becoming a playable character in the next Smash Bros. But definitely second to Bowser Jr., as his popularity has definitely escalated with the help of his numerous appearances in recent Super Mario titles.
Characters that I would like to see join the fray, would have to be... well... let me categorize it a bit;
Pokemon Series:
Zoroark (A popular Pokemon introduced in the most recent generation. An easy replacement for Lucario, just as Lucario replaced Mewtwo)
Pokemon Trainer (With different starter Pokemon, of course. Possibly using 2nd to 6th generation starters. That is, if the full evolutions to the 6th generation starters are revealed during development)
StarFox Series:
Krystal (Easy way to shovel in another female character in the male-dominated series of Smash Bros. Also a potential trigger to some more recognition for a somewhat dying franchise)
Super Mario Series:
Bowser Jr. (Clearly a popular enough character to earn a spot in the next Smash Bros. installment. Also carries a lot of potential for an interesting moveset)
Waluigi (Same reason as above, though his current listing as a "spin-off character" hinders this possibility quite a bit)
Donkey Kong Series
Dixie Kong (Definitely would make for an awesome entry in the DK series of Smash Bros. And if anything, would definitely be the next character added to the DK division. At least... I think so)
Can't really think of any other characters that I would like to see make it into the next Smash Bros. as I'm tired of thinking... and typing.