Waluigi is already playable, because Waluigi IS Mario.
You see, the Super Mario series is nothing but excuses ultimately made to give Nintendo money.
It's almost like the body for example
There's the skin: Mario; the cheeky, happy-go-lucky plumber, who has no depth whatsoever
take that away and you get something similar, but less: Luigi; a Mario with depth, but only to act as a reflection to Mario
dig a little deeper and you find an invert of the skin: Wario; a character with absolutely no backstory... but is Mario's needed rival
then take all of that away and you are left with the heart: Waluigi...
Mario, Luigi, Wario, Peach, Toad, Bowser, and everybody else are excuses to make the Mario universe seem like more than what it is: a money machine. You get sold the same game and the same content over, and over, and over, and over, and over, and over again, but after awhile you start to catch on... what if there is something more? why is this the same game?...
Because Waluigi. Within that dark spot in the Mario character's eyes, within the coding of every game, within every dollar bill, pixel and bit, variable and hex value, is Waluigi.
Waluigi is a character advertised to you for the sole purpose of getting money, he is the Mario with no more excuses, no more get-arounds, no more quirks: Waluigi is the truth. Waluigi has no depth but all of it at the same time. Waluigi has no backstory, he is the reflection of a reflection of a reflection; or perhaps, the Mario characters are a reflection of him. Waluigi is real, look at Mario, Luigi, and Wario, they are all around the same height, then look at Waluigi, he isn't even Luigi's reflection, he's a real man. Everything in the Mario universe, from the Goomba, to Daisy, to Toadsworth, to Princess Shroob even.. Is all Waluigi. The Mario World is nothing but an illusion for Nintendo to gather a quick $, and who loves money more than Wario and Waluigi? The only problem is Wario has some character, he loves money, hates Mario, is friends with Waluigi, and has a pretty quirky playstyle from the Warioware franchise to Super Smash Bros. but Waluigi? He is just there because.. he is..
that's it.
Waluigi is Mario and Mario is a tool for a quick $.
now you know...