I agree with this thread, though about characters I think that Magolor would be a better representative of the modern era than Bandana Waddle Dee.
Another Dimension and the ancients have been a major plot/lore element of all the modern Kirby games (excluding maybe Triple Deluxe, where anyway Another Dimension was still a Kirby Fighters stage), and while Magolor (who is directly associated with Another Dimension) has been playable only once, he's still considered as an important character for his own merits and not just for being playable (he's kinda the Rosalina of Modern Kirby; Another Dimension is to Magolor what the galaxy/gravity gimmick is to Rosalina).
If you ask me, in order to represent the modern era, I would still put Bandana Waddle Dee in the game, but as an assist trophy, replacing Knuckle Joe with him completely, and I would put Magolor in as playable instead (as much as the fans want the other way, I think that this is the options that makes the most sense).
Also there is nothing in Smash that represents Kirby's "art" games, such as Dream Land 3 and the crayon graphics, Canvas Curse's art style, Rainbow Curse and the clay world, and Epic Yarn's fabric. That has been a major recurring element in the series and it's not represented... Yoshi has his world of wool and cardboard represented in Smash, the Woolly World and Yoshi Story stages, why can't Kirby have a clay stage or a paint stage from Canvas Curse, or that cursed Epic Yarn stage that got scrapped?
And speaking of the Dark Matter trilogy, the factory on Shiver Star (Kirby 64) is one of the most iconic locations from the Kirby series; you can argue that it's a mechanical stage like the Halberd, though it's still a very iconic and well known location, and its music is even more iconic and recurring than the location itself. Why it's not represented in Smash yet?
Also at least one character from the Dark Matter trilogy should definitely be playable; we need modern Kirby representation offcourse, but the Dark Matter trilogy is half of the Dream Collection:
the other half being more than well enough represented in Smash already.
(I'd definitely go with Adeleine for this, though the animal friends are also a good option, considering how they were also featured in spin-offs such as Star Stacker. I prefer Adeleine though because with her drawing skills she can represent many more characters and game elements - potentially including the Animal Friends and Dark Matter - simply by drawing and summoning them).
An art-related stage, Another Dimension and the Factory are IMO the biggest lacks of Kirby representation in Smash so far (stage-wise), and we were very close of getting an Epic Yarn stage to fill the art-related slot; we got a Yoshi Story 3.0 stage instead.
Then, a character from the Dark Matter trilogy (+ 1 assist trophy and other content representing the trilogy) and a character from the modern series (+ at least 1 assist trophy and other content to represent the modern games).
This content should have been added starting with Smash 4, not all in one game; yet, in 2 Smash games, we have got nothing of this.