Hear me out - Why not both? I think the Camelot Bros both have a valid shot, and valid reasons, for being upgraded to a playable status.
Let's factor in a few aspects of DLC that we know to be the case:
Characters must come with a stage, a DLC Spirit Board and a selection of music.
Characters must bring something unique to their playstyle that differentiates themselves from the base roster.
The only exception to this is Plant.
If you look at every DLC fighter except Plant, all of them adhere to that second requirement. Joker has Arsene, Hero has the Command Menu, Banjo has the Wonderwing, Terry has Go Sauce, Byleth has weapons tied to different directions in a way we hadn't seen before, Min Min has her insane reach & unique specials & Steve has the Mine/Craft/Block Place mechanic.
Now, for Isaac, he could easily have a unique "mechanic" that revolves around Djinn, summons or just general psynergy attacks. In Waluigi's case, I'm not certain as to what unique "mechanic" he would bring to the table, there's nothing that really "stands" out in that aspect.
I'd be more inclined to believe Waluigi was a "bonus fighter" similar to Plant, if he does get in. No hate to Waluigi, far from it - I just don't really like the idea of paying for a DLC where the only spirits we get revolve around Mario Sports. Feels like a step down from pretty much every other DLC spirit board, in that sense.