Smady
Smash Master
This guy has been in more games than can be counted on fingers and toes, dating all the way back to the first Mario Tennis game - that's right, he's older than Bowser Jr.!
His main competition for a Mario series representative is arguably Bowser Jr. and Toad - the former of which is a character who appears as Bowser's minion, and Toad who is already part of Peach's moveset and would be difficult to give one of his own. Paper Mario has as big a shot as Waluigi, but he represents his own series. Waluigi represents the entire spin-off lines of games and could have a very wacky and original moveset based around rackets, clubs and other sports apparatus. He was even an assist trophy in Brawl, unlike other competition, showcasing the appreciation Sakurai has for the characters and its fanbase. Sure, he has his haters, but you can't deny his presence in Smash would be a unique one.
Let the speculation begin!
DON'T LIKE FUN? TOO BAD, WALUIGI TIME!