So because of all the fan demand it's a strong possibility that they start to pay attention to all the Waluigi requests.
I am not entirely certain of his combat abilities or why 'he's so popular. Would he use a lot of stretchy punches and kicks, like Dhalsim from Street Fighter?
Technically it's not
impossible for that to happen, but that would really take away from the potential ARMS character. And honestly, his long legs already have good reach; I don't think he
needs to have stretchy legs.
Waluigi is honestly a blank slate in that almost any sort of random power you could think of to use as a fighting technique would fit him considering that's exactly how he's treated in the games he's in.
One game, he's creating tornados and turning his side of a court into a personal swimming pool, the next he's spawning purple thorns from the ground and teleporting in puffs of purple smoke, and the next, he's firing energy projectiles that have his insignia, and so on and so on.
Hell, even in written media, Waluigi has done things like drill deep holes into the ground just by spinning in place (trying to get at something on his back, I think) and climbing his way back out using his
own chin as a pick.
The only truly
recurring ability is his ability to defy gravity and swim through the air, which was something he was shown to do since his very debut in the "No Controller Detected" error screen for Mario Tennis.
So that's the beauty of Waluigi's potential fighting style; there's no set "obvious" answer.
Even things that are just simple quirks about him could be weaponized if Sakurai sees fit.
That common thing where his eyes shine could be turned into some eye laser ability.
His affiliation with roses? Go all Tuxedo Mask with them.
His common usage of bombs and other explosives (such as the Whisker Missle from the Namco-made Mario Kart games)? Make him a Demoman kind of character.
There's so much that can be done.
Though we already know one thing for sure. Based on how his Assist Trophy works, he would be given focus on a Tennis-based style along with very high jumps and strong kicks (which isn't an "ability" per se, but is something Waluigi is known for in the Mario series, even knocking
Bowser down in a brawl with a mighty kick in Mario Party 3). And that's just on a basic level.
The rest comes from whatever Sakurai would decide on how Waluigi should fight.