I don't know, I see Waluigi as an Assist Trophy that doesn't have any strong identities with music/stages/moves. Really all he has is memes.
All the DLC picks for Wii U had really strong identities attached.
Ryu, Bayo, Corrin, and Cloud all have tons of detail thrown into their design and representation.
Waluigi is just "meme".
You're forgetting Mewtwo, Roy, Lucas, etc.? AKA the initial batch? Not exactly brimming with "strong identities" as far as Smash movesets go, but they were fan favorites that were easy to implement as the first couple DLC packs because their movesets were already defined by past games. Assist Trophies aren't the same, but some work on their assets, animations, damage, hitboxes, etc. *is* done.
The only thing screwing Waluigi over is his Assist Trophy status, but in code there's no reason not to have a line like
if (is_any_player_using_waluigi()) {
disallow_waluigi_assist_trophy();
} else {
dont();
}
Or fuck it, don't even bother. This is the same company that concocted "Goouigi" for Luigi's Mansion 3DS multiplayer. I doubt they care much about "canon" in multiplayer, so who cares if there's a Waluigi trophy and Waluigi player running around at the same time? You can put 8 of the same playable character on-screen at any time.