In literary terms, a foil is a character that is intended to act as a contrast to another character. Nintendo’s mascot, Mario, has two notable foils in the form of Luigi (his brother) and Wario (his “bad counterpart”). The year 2000 would see both Luigi and Wario receiving a mutual foil in the form of Waluigi, who made his debut in the Nintendo 64 game, Mario Tennis, which was released in North America on August 28th of that year (the game was released in Japan on July 21st of that year).
While Waluigi has existed for 25 years now and has consistently appeared in games since his debut, it hasn’t been lost on many that he still has yet to appear in a mainline Mario game. He also has still not yet become a playable character in the Super Smash Bros. Franchise, for that matter. Over the years, I’ve seen many possible reasons given for why the Mushroom Kingdom’s purple and black clad plumber has yet to “formally” appear in an “in-house” 
Nintendo game (not counting Easter egg acknowledgements, such as in Super Mario Odyssey) but was never able to figure out which reasons were the truth and which were merely speculation. Regardless, I’m hopeful that Waluigi will have his chance to appear in a “major” game within the Mario Franchise, eventually. That hope has lead to me having some thought exercises on how he could appear in a “major” game.
To that end, I would consider a good first step would be to take a page from both the characters he acts as a foil to, Luigi and Wario. To elaborate, Luigi had been one of the playable characters since the early mainline Mario games that featured multiplayer options in some capacity. Wario gained his own spin-off series (which is split into two sub-series in the form of Wario Land and WarioWare) a little over a year after he made his debut as the main villain of Super Mario Land 2: 6 Golden Coins. I also think it’s worth noting that the only mainline Mario game that Wario has been playable in by this point in time is the Nintendo DS remake of the iconic Nintendo 64 game, Super Mario 64.
Taking these factors into account, I would consider the best first step to giving Waluigi an appearance in a mainline Mario game would be to have him appear as a playable character alongside Wario within the spin-off series of the Mushroom Kingdom’s yellow and purple clad plumber (who is also known to wear a biker getup at times). Due to the WarioWare sub-series consisting of games that are, essentially, a collection of mini-games, I would consider the Wario Land sub-series to be the preferable sub-series to integrate Waluigi into Wario’s spin-off series to make use of gameplay potential. Considering how Waluigi’s built is an exaggeration of Luigi’s built, I could see his gameplay involving him having less strength and durability but making up for it with greater speed and jumping height when compared to Wario. I would also be on board with it being a 3D game since the previous (and so far, only) time Wario had a 3D adventure of his own was Wario World for the Nintendo GameCube.
Of course, this is merely the tip of the iceberg for a starting point to give Waluigi a more prevalent role in the greater Mario Franchise. Some might say that a quarter of a century without a “significant” role stacks the deck against Waluigi but the recently released Donkey Kong Bananza gave Nintendo’s big ape another 3D adventure during the year of the 26th Year Anniversary of his first 3D outing, Donkey Kong 64. It has also been said that crew of Bananza consisted of younger developers who grew up on the DK games developed by Rareware which seems safe to say was a likely factor behind some of the concepts and aspects that were included in Bananza. This helps reinforce my hopes for Waluigi having a chance to play a “significant” role as developers who grew up on the games featuring Waluigi join Nintendo’s ranks. At the very least, I’m hopeful Luigi’s Wario will become a playable character in the Super Smash Bros. Franchise before Waluigi’s existence reaches the halfway mark of a century.
