I'm gonna regret saying this, but that might be in Waluigi's favour... Waluigi needs no introduction, everybody knows him and his history.
I agree, you would expect at least a 25 mins presentation for a lot of characters whether because not a lot of people know where they come from or because they are icons with some massive history. Even Banjo Kazooie got a 25 mins presentation because the younger fans might not be familiar with them... but Waluigi, everyone and their mother know him, he's a mainstream character. When you have Netflix and The Washington Post talking about him, you know you don't need to ask who he is.
And besides, what would they say about him ? "Waluigi debuted in 2000 on Mario Tennis for the n64. He's Wario's friend and claim to be Luigi's rival. He appeared on multiple spin-offs since then." That's it, 3 sentences to describe him, that would be kinda weird for a 20 mins video. It would be the same if he presented the spin-offs.
"What is Mario Tennis ? The first game of the series came out in 2000 on the n64. In this game, you play tennis with the various Mario characters including Waluigi. Mario Tennis games were produced on every consoles since then including the Switch with the lastest installment of the series, Mario Tennis Aces."
They don't need a lot of time to explain who Waluigi is. He is extremely well-known already and it's weird to say that but he's fortunately not important enough to give time to talk about him. he is just some popular mainstream character from Mario games and it's perfect.
I guess we will see the Waluigi trailer, and then he will joke about how much people wanted him before giving infos for the future like if cocovirus will delay the rest of the pass.