if Nintendo decided to give Waluigi his own title.. what would it entail?

Following the events of Waluigi Unleashed III, Waluigi is hiding in the desert, having successfully avoid 64 assasination attempts by Lanky Kong's death squads. Hearing rumors that a resistance by loyalists of the former Primate Alliance junta still exists deep in the jungles of the Banana kingdom, the man goes forth, eager to bring ruin to his worst enemy.

Waluigi arrives in-extremis to save the resistance and its leader Colonel Bluster Kong from annihilation by the Party Monkey spec ops, but both parties are surprised by the arrival of an unexpected interloper: Mr. Eraser.

Mr. Easer presents himself as the sole survivor of the Fifth Dimension, which was entirely destroyed by Lanky Kong in an event know as the Genoblast. He then goes to explain that from a single dimensional genocider, Lanky Kong is a "dimensional cancer", an entity whose various multiversal permutations have caused untold deaths and carnage. Waluigi volunteers to destroy all incarnations of Lanky Kong and maintain the stability of the multiverse.

Traveling through dimensions and fighting foes such as Pink Gold Lanky Kong, the dissapointing robotic dupplicate Janky Kong and the martial arts expert Lanky Long, Waluigi absords enough of the slain Lanky Kong souls to unlock their power and storm Lanky Kong's fortress, Blue Banana.

Finally reaching the top and subduing Lanky Kong in a shitty and frustrating series of quick-time events, Waluigi prepares to execute the orang-utan until Lanky reveals the truth: Mr. Eraser wasn't entirely correct in claming to be the sole survivor of the Fifth Dimension: Lanky Kong was native to the dimension and settled in the current one after. Thus, he is not this dimension's native Lanky Kong; rather, Waluigi, pointing out to their ability to stretch themselves for the pleasure of the audience, lack of style, abscence of grace, and funny face. Despair soon takes over Waluigi.

As the screen fades to black, the sound of a gunshot is heard,
Regardless of what kind of game it would be, the plot should be about Waluigi being humiliated and laughed at by the other bros for doing x thing badly and you have to prove them wrong by mastering doing x thing and becoming better than them.

I'd like the game to be a dancing one. If that's too much to ask for make it a tennis one instead.
Regardless of what kind of game it would be, the plot should be about Waluigi being humiliated and laughed at by the other bros for doing x thing badly and you have to prove them wrong by mastering doing x thing and becoming better than them.

And then it's revealed at the end they could still beat him, they just took pity and let him win.
Wario and Waluigi get wind of some massive fortune hidden away somewhere and they head off to claim it for themselves. Upon finding the treasure, it entices them to turn on each other and begin fighting over it. Wario manages to gain the upper hand and takes the treasure for himself, only for it to disappear--- the treasure was never real, it was only a trap meant to enslave the one greedy enough to grab it! Wario, now a slave to the mysterious figure who set the trap, turns on Waluigi-- not his usual bumbling self this time. Waluigi is swiftly chucked across the world to a far-off uncharted wasteland...

Thus begins Waluigi's trek across the weird unexplored outlands of the Mushroom World. To save Wario---not for Wario's sake but to be praised, and maybe he'll even get clout from the Mario Bros! With delusions of sniping the ultimate glory from those do-gooders, Waluigi's head is held high as he faces whatever might stand in his way. Little does he know, the evil at work has done some... twisted things to the Mushroom World, and more major players are going missing. Mario, Luigi, even Bowser... Soon, there won't be anyone left to watch Waluigi save the day. Hey, that's-a cheating! WAAAAH!

Waluigi's Wicked World

Coming April 31, 2020
Waluigi and Wario get abducted by aliens and are forced to participate in some kind of intergalactic gladiator game show that is being broadcasted across the entire universe. Initially they are both pretty reluctant and irate about the whole ordeal-- that is at least until the (totally not at all suspicious or sketchy) host reveals the fabulous untold riches and renowned glory that winners shall receive. Frothing from the mouth from such glamorous rewards, our dynamic duo do a heel-faced turn and are suddenly on board with the idea.

Ideally it would be a 3D beat-em-up with platforming elements similar to Wario World, the player can freely swap between controlling Waluigi or Wario (or even do co-op) at any time. Wario relies on his brute force to take down foes whereas Waluigi uses a myriad of different gadgets each serving a different purpose. During normal stages there will be plenty of treasure and goodies to collect and depending on how much you collect by the end of the game will determine what ending you'll get.

Also Tatanga appears as a optional super boss.
"Psycho Waluigi is a fangame made by Thunder Dragon of MFGG fame. Set in the Mario Universe, it stars Wario's underappreciated partner, Waluigi.
After taking a nasty fall from his hot air balloon, Waluigi wakes up in an unfamiliar land with a strange, purple, eyeball cloud watching him. The eye identifies itself as Psycho Iris and tells Waluigi that he is in the land of Unconcia. He goes on to explain that due to both the natural influence of the land and his own phenomenal power, Waluigi now has psychic powers. Now that he has his own superpowers, what will he do with them? Save a princess of his own?
Nah, he's gonna go take out the Hazelnaut Army and conquer Unconcia for himself.
The gameplay is like a cross between Wario Land and Psychonauts, with classic platforming action. Using Psycho Iris, Waluigi can use Psychokinesis to weaponize the environment and his enemies. For example, by lifting up a fireball-spitting Piranha Plant you can force it to shoot out fireballs for you."

-TV Tropes
tbh, he won't get one since Nintendo still sees him as just Wario's sidekick but at the same time doesn't want to use him in Wario's own games.
I think since he's clearly inspired by Dick Dastardly, a game based on the Flying Machine series might work.

Unlike Wario, Waluigi is still very much a bad guy in his game and doesn't do anything noble at all, he's a jerk in a plane causing trouble