For what it's worth, I've always felt like Zelda and Kirby were not selected randomly to be the two Nintendo series to receive a new boss in Ultimate.
Kirby in particular, is the only core series (with core I mean those that have been in since the first couple of games and aren't limited to just one character) that didn't receive new content, or even any "old" new content. Zelda did get more new stuff: a new stage, a couple of estensive reworks to existing characters, and of course the return of Young Link, but it's not quite like getting K. Rool or Ridley or Incineroar.
Not saying that Ganon and Marx were consolation prizes, but I don't know. To me it felt like they knew people wanted more Zelda and Kirby characters and acknowledged that via a boss. Like, I don't know, they have a similar energy to K. Rool's Mii costume back in Smash 4 imo.
(By my own definition, Starfox also didn't get anything, but being a much smaller series than Zelda and Kirby nowadays, Wolf coming back felt to me like it was enough.)
So yeah, it's a gut feeling but I'd keep an eye on those two. They're very likely to get a newcomer next game imo.
Speaking of bosses, I went back to Pyrosphere Ridley after all these years out of sheer curiosity and lmao. That really didn't work as the "correct" Ridley representation as opposed to his playable form.
He floated around slooooowly, spent half his time crashed on the ground due to how many hits he took from all the fighters and getting up slooooowly, and sometimes he hit the stage with a couple of slooooow screen-filling attacks.
When I think of Ridley boss fights, I think of frantic encounters where you're both constantly moving and he can drain your health in a couple of attacks, size aside Ultimate Ridley captures that energy so much better than Smash 4's giant Ridley pinata.
The Brawl boss fights, while still not ideal, also managed to make him feel like Ridley, the second one in particular... But man they really dropped the ball with Pyrosphere Ridley imo.