Honestly, i think the only major contender Waluigi has in his Smash inclusion is Bowser Jr, since Jr is the only character left to represent the main series ( Super Mario Bros/3D games/New). With Mario, Luigi, Bowser, Peach, Toad ( in a non-playable way) and Yoshi ( who represents his own series, but is still a Mario series) already in, the only ones left would be Nabbit, Birdo and the Koopalings, who have as much chance as Master Chief, and Jr. Jr is a reasonably big Mario character, since he often acts as Bowser ´s second in command. The next biggest selling branch of Mario are the spin-offs. That´s were Waluigi comes in.
But, when it comes down to who vs who, it´s not for us to choose who will be in Smash. It seems that Nintendo is picking characters for Smash based on their recent games. Besides the 7 first veterans, Megaman (who was long overdue for an appearence) and Wii Fit Trainer ( who came right the **** out of nowhere), we have: Villager( AC: New Leaf), Toon Link ( A Link Between Worlds), Marth( Fire Emblem: Awakening), Peach (Super Mario 3D World), Olimar ( Pikmin 3), Bowser ( he is a mainstay now, but his design takes cues from Bowser´s Inside Story), Luigi ( a veteran, but revealed early because of The Year Of Luigi), Pit( Kid Icarus: Uprising) and Sonic ( Sonic Lost World). Sakurai could add Jr because of his longtime requests and New Super Mario Bros ( Luigi) U or Paper Mario: Sticker Star. He could use add Waluigi because of his longtime requests, his early appearence in the series, and Mario Kart 8 or Mario Party: Island Tour. I feel like Nintendo is adding characters that had recent appearences. I could be wrong though. This is pure especulation.