Well, assuming Waluigi is indeed Fighter 11, then hypothetically what could be taking longer?
1. Yeah, the Assist Trophy thing would likely be acknowledged in some way. Turn it off when Waluigi is there, let it stay, replace it with a different character, there area good few options here, maybe even a few I haven't considered but yeah that's the deal and I'm sure they'd have to sort it out.
2. THIS IS A BIG THEORY that those in the speculation thread have heard me regurgitate a couple times but hear me out!
What if Waluigi's stage is NOT Waluigi Pinball, but Waluigi's Island from Mario Party 3?
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This would not only be a Waluigi stage, but ALSO double as a late Piranha Plant stage, and TRIPLE as "at long last, Smash has a Mario Party stage!".
And IF Waluigi's Island returns in Mario Party Superstars, then this would also quadruple as a promotional pick for MP Superstars!
And IF Waluigi truly is coming with more Mario Party content (the game as is I BELIEVE has one Mario Party song and like, 3 Mario Party specific spirits, two of which were the first post-release spirits. Collectively those spirits are MC Ballyhoo/Big Top, Dice Block and River Survival), then MAYBE Mario Party is incorporated into Waluigi's moveset, and THAT'S what's taking so long.
Like... Waluigi's moveset has all the friendship ruining elements of Mario Party in there somehow, making him the ultimate Mischief/Prankster/Troll character. Maybe he's got the tennis racket/golf club for some of his attacks sure, but his specials could well be Mario PARTY based and have some randomized element to it or some other Mario Party type thing.
Mario Party has been everpresent throughout Smash's lifetime, and yet it still has no meaningful content in Smash beyond A song and some trophies/spirits. Perhaps Waluigi will change that?
Also HI EVERYBODY! I support Waluigi and am willing to bet my Burger King that he is indeed the 11th and final.