For me, the most likely contenders would be these five, in no particular order:
- Duck Season from Looney Tunes: Mario and Wario argue with each other over whether it's "Italian Season" or "American Season" for Bowser (ripping labeled-as-such sheets of paper from a ridiculously thick stack that just so happens to be nailed onto a tree stump while doing so), then Wario asks Bowser if he wants to "roast Mario now, or wait until he gets home"
- The Main Characters from Beavis & Butthead: Wario would become Butthead, while Waluigi would become Beavis; basically, the main formula for Devious & Butt-Chin's episodes would consist of the two of them sitting around in their house all day and bitching about whatever random video game (usually a very pop-culturally famous one) they happened to find themselves playing next, as a blatant satire of YouTube media critics; whenever they weren't doing that, they would be going out into Diamond City to steal money, hit on random girls, and generally be trouble-making nuisances, just like the original Beavis & Butthead
- Robbie Rotten & Stingy from Lazy Town: Waluigi would play the role of the former, while Wario would play the role of the latter; Luigi would become Sportacus, Stephanie would become Teenage Kat, Lazy Town would become Wario's revised name for Diamond City, etc
- Di Lung's Catchphrase from Courage The Cowardly Dog: The best possible scenario for this would be as follows; Mario and Luigi are busy driving around the Diamond City outskirts in their rusty, banged-up old pickup truck, when all of a sudden, Wario (driver) and Waluigi (passenger) abruptly pull up right in front of them (using a T intersection, no less) in the futuristic hover-car from Wario Land 4's best ending, horizontally blocking the road that Mario and Luigi are driving on and therefore nearly causing them to crash. Naturally, Waluigi's response to this is to angrily shake his fist at Mario and Luigi and (extremely hypocritically) yell "WATCH WHERE YOU'RE GOING, YA FOOLS" at them while Wario flips his engagement-ringed middle finger at them and spitefully laughs at them, thus provoking Luigi into shooting such a downright nasty death glare at Wario and Waluigi that it causes their car to explode into smithereens while Mario clutches his head with both hands and screams "HOLY MEATBALLS" in response (the second best possible scenario for this would be Waluigi formulating his evil plan to prank Luigi on a portable markerboard while standing right in the middle of an open road, then getting cartoonishly flattened into the pavement of said road by Wario's passing, honking delivery truck and even more hypocritically yelling "WATCH WHERE YOU'RE GOING, YA FOOL" at him in response)
- Stimpy's Fan Club from The Ren & Stimpy Show: The entire episode becomes Wario's Fan Club, with Waluigi being officially declared the President of Wario's titular fan club by no other than yours truly...which initially sounds like extremely good news to Waluigi, until he finds out that what it really means is that he has to sit at a boring old desk all day, 24/7, sifting through Wario fan letter after Wario fan letter, progressively becoming more jealous of Wario's popularity with each one that he reads...until finally, near the end of the episode, Waluigi officially loses his mind entirely and goes off on a hilariously melodramatic Shakespearean tangent about how he is going to murder Wario in his sleep with his bare hands in a fit of sheer-envy-induced rage, just like in the original episode ("He's a JERK...FAT, GREEDY LITTLE JERK...how easily I could end the farce...with THESE hands...THESE...DIRTY...HANDS...AND with these hands, I hold the fate of MILLIONS...they think he's a GOD...BUT HE'S AS MORTAL AS WE...JUST...ONE...QUICK...TWIST...AND IT'S OVER...JUST...ONE...")