That debate is only relevant to the protagonist first "rule". Kirby's already in Smash Bros., and the door's wide open for subsequent reps from a series to be supporting characters. Whether Nintendo actually wants to sell a supporting character as DLC is another thing entirely and remains to be seen...
So we know that Nintendo is essentially only choosing characters who are main characters for DLC. (Even in Fire Emblem's case, they promoted a new game with a new character as the main character of that respective game). ARMS introduced a case in which "everyone is the main character"; therefore, it was easy for Nintendo/Sakurai to pick any character that they wanted. Like someone mentioned earlier, this doesn't apply to any other franchise.
Case in point, Terry was specifically chosen as the rep for Fatal Fury/KOF without any mentions of potentially choosing anyone else besides him.
In terms of the Kirby series, it's pretty clear that Bandana Dee has established (and is continuing to establish) his role as a main character within the franchise.
Therefore, I believe that he still has a chance
but only under certain conditions which have to line up
perfectly in order for someone like him to even possibly be considered:
- The release of a Kirby game conveniently falling into the window for the DLC period
- Bandana Dee having a significant playable role within that game
I mean, he doesn't even need to have his name on the box (as proven by the names of other franchises/games), but he should be someone who you could pick up and play with ease so you could get to know him over the course of the game.
That's also why I'm unconvinced that characters like Marx would be chosen if they didn't have a significant playable role.
Dare I say it, that's why I believe that we won't see a single villain for the rest of the DLC because of the focus on significant,
playable roles.
(Unless said villain is conveniently and notably playable throughout the course of a certain game).
Also, the whole "Super Smash Bros. Ultimate X (Franchises/Game)" could easily be fulfilled by specifically promoting the newest Kirby game (akin to Fire Emblem: Three Houses being the newest Fire Emblem game). "Super Smash Bros. Ultimate X Kirby" simply wouldn't do.
The next Kirby game is supposed to be the "pinnacle of the series" anyway, so who knows? Maybe Nintendo would want to take advantage of that if Star Allies didn't perform well enough to their liking.