I don't think either are particularly in need for. We have plenty of Sonic Spirits and enough music, Modern Kirby has plenty of Spirits & Music as well as the series itself having a lot of stages, which if Sakurai had wanted a modern Kirby stage, he'd have put one in by now, he probably just prefers using his characters and games that he is more familiar with.
Bandanna Waddle Dee just doesn't feel 'big' enough for DLC, sorry to say, he is more of a Base Game pick.
Note: I know Piranha Plant is DLC, but given it was a timed free character and has a Palutena's guidance, it may have been intended for base game or as a character to give as a bonus since few would feel it was worth paying for or getting much hype, plus it didnt come with spirits, music, or a stage further separating it from other DLC fighters.
I would argue that the ratio between Classic and Modern Kirby content, while not entirely abysmal, is still irritably one-sided; which is especially strange for a thriving franchise like the Kirby series. Assuming that the core Modern Era of the Kirby franchise started in 2011 with Kirby's Return to Dream Land, I could only pinpoint about 9 Modern Kirby spirits in total (disregarding important characters like Taranza, Star Dream, or Hyness). On the flipside, I believe that the music selection has at least improved even if there are still some strange omissions like "Galacta Knight's Theme" and "Masked Dedede's Theme"; however, it's a little unfortunate that there still isn't one remixed Modern Kirby track to be found within the entire selection.
When it does come to more noticeable elements of representation like Stages and Characters, I do want to cut Sakurai some slack.
Back in Smash 4, he did consider putting in a Kirby's Epic Yarn stage, although I would assume that the newness of Yoshi's Woolly World inclined him to develop a yarn-themed stage with that game in mind instead (not to mention that it saved the Yoshi series from having 0 new stages in Sm4sh).
Now in Ultimate, there wasn't a single 1st Party franchise (aside from Mario and Zelda) that got a brand-new stage in the base game due to the development plan of including more existing stages in Ultimate above all else. Therefore, it's hard to judge whether or not Ultimate would've been the game in which he finally put a Modern Kirby stage into the series.
As for characters, it certainly could be argued that Bandana Dee could've (or even should've) gotten into the base roster of Ultimate, and that it's simply too late for him now in a world where DLC characters depend on how much they can turn Nintendo a good profit. Personally though, perhaps one of the only things holding him back is something I would call
"Rising Star Syndrome" because of his significant prominence in the Modern Kirby era in comparison to the Classic Era (where Kirby, Meta Knight, and Dedede are prominent in, as well as the Modern Era). Since, subjectively speaking, I don't see why Bandana Dee wouldn't be considered a 'big' enough pick for DLC at this point. As of now, it's clear that HAL has been trying to push him as a main character ever since Kirby's Return to Dream Land, effectively separating himself from the average Waddle Dee/Servant of King Dedede while becoming his own distinct entity in terms of weaponry and personality. Not to mention that he's also basically the sole representative of the Waddle Dee species, and would serve as a beacon for that species, which has been with the Kirby series since the very beginning, in a game like Smash. Also, generally speaking, Waddle Dees are as marketable as they could get, especially in a country like Japan.
Now, another thing potentially holding him back is the lack of a game to promote (which could possibly be resolved if a new core Kirby game managed to release this year, of which he would then have to star in), along with the reasonable argument of whether or not Kirby could simply promote the game on his own (like he typically does). However, as a prominent figure in the Kirby franchise, I personally don't think it would necessarily be a horrible business decision to make him into a DLC fighter for Ultimate either way.
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