It doesn't make Sense at all...
They already have a role in the game.
No need to change them...
In all fairness, DLC has more or less been its own thing in Smash 4 rather than an extension of the base game.
I mean, both Mewtwo and Lucas had trophies in the base game, but that didn't stop them from being DLC nor did it remove the base game trophies we already had. Of course, regular trophies and Assist Trophies are far from the same thing, but it
does show that base game content does not necessarily deny characters the chance of being DLC.
And Waluigi's demand since E3 is, to put it simply, undeniably high, so making him a DLC would definitely sell well. On top of that, he already has a model and some animations, so he'd be easier to make than someone who has to be made from scratch. Less effort for more money. An
ultimate combination.
With that said, do I think it'll happen? Probably not, but it's still an option that
can happen.