I'm making a post that detracts a popular character, I'm sorry, I'll try to make it as much objective as possible and with documentation and explanation, so it's not an hate post (it's not my intention).
Are you sure about that? Does he?
I mean, I agree about it being popular and having a lot of fans, but is he really on the same level as the regular cast?
Ok, in Kirby Star Allies (Guest Star) he has the same intro picture as Metaknight and King Dedede, ok... but that's a weak point IMO, he's there to represent all the waddle dees (according to the description), and he has been in many games, he doesn't have a specific game to represent so they went for the neutral screen (the other characters have a screen that emulates the boxart of the games they are from).
But what about his presence in the other games? Most games he is in are spin-offs, and most of the times he gets a side role. Team Kirby Clash Deluxe is the only game where he gets a slighty bigger role as he's along with Magolor the only NPC you can talk to. But in the 3DS games he's just there to throw stuff at you and does not appear in the story, in Superstar Ultra he is a minor character of a side mode; in the anniversary collection he just appears and does nothing.
Now let's talk about those games where he apparently had a major role:
Kirby's Return to Dreamland: He's one of the main four characters, though he barely interacts with the other characters, just as anyone else. Metaknight, King Dedede and Waddle Dee are there to allow multiplayer, story wise they just do the same actions as Kirby and stare; they aren't really involved in the story. I think Waddle Dee (so it was called) was there more to replace a proper helper system than to be its own character.
Kirby Battle Royale: He appears in the story mode, he's a main character in that mode, though.......... when you play multiplayer (the main mode of the game, as it's mainly a multiplayer party game) you are not allowed to play as him, you can only play as Kirby, Metaknight, King Dedede and a Waddle Dee without the bandanna.
Now about Rainbow Curse:
this speaks itself. Bandana Dee is not even considered a character, it's mentioned as an helper of Kirby but he's clearly intended to be a minor character.
Lastly, HAL almost always prefers to use regular waddle dees instead of Bandana Dee, in promotional material and merchandise. They made an amiibo of Waddle Dee, that's just a regular Waddle Dee.
So I wouldn't call him part of the main cast. Surely he appeared a lot of times, but he has always been a secondary character, most of the times a minor one too. Magolor on the other hand, appeared less, but has way more involvement with the lore of the series, and his action had some impact too.
While Magolor is not my biggest priority as a Kirby newcomer, I think Magolor is a lot more relevant than Bandana Dee.
It seems that HAL considers the main group of characters to be Kirby, King Dedede, Metaknight, and Waddle Dees. Waddle Dees as a species, not as a specific character. But when you think about it, does a regular Waddle Dee really deserve to be in Smash? I don't know, really. For sure I would prefer a regular Waddle Dee, with a bandanna as one of the many costumes (along with the sailor hat and other stuff), than Bandana Dee as a specific character in the game ond only palette swaps as costumes (also if you take Waddle Dee as a species, you have way more freedom to build a moveset, taking inspiration from a lot more source material than just the Spear Ability and to an extent Parasol).
I agree with you about it pleasing the highest amount of fans, that's a good point, but I wouldn't bring relevance into this, it's more about granting the fans' requests; it all depends on if Sakurai thinks that a Waddle Dee deserves a spot in the roster or not (if he thinks it does, then expect him as a newcomer, but expect him to be Waddle Dee, not Bandana Dee; but if Toad is of any indication, then I wouldn't bring my hopes too much up).
Anyway, there is a rumor about the voice actress of Kirby, apparenly she has replied to a Bandana Dee fan with a vague comment that seems to hint that Bandana Dee in Smash is a possibility, so yeah, it's a possibility, but I still think it's only due to fans' request, not because of relevance. And nothing is confirmed anyway, it's just a vague rumor.
If all those fans who say "I like that Kirby character but Bandana Dee must be in first" would have supported their actual favorite character instead, it would have had an actual chance instead, this way IMO is just a wasted opportunity for all the Kirby fans.
