People who aren't as familiar to the Kirby series seem to freak out when more than one Kirby newcomer is proposed. These are my top picks and here's why:
1. Bandana Waddle Dee: Obvious choice, the only one that really doesn't need explaining.
2. Magolor: He's very popular, especially in Japan (he placed 7th in the fighter ballot), and aside from BWD he was also the face of the new resurgence of Kirby games after 2011. Magolor has appeared in or at least made cameos in almost every Kirby game since. He would have a unique moveset to go with it.
3. Marx: Marx is arguably the most famous and iconic Kirby villain. He's also one of the series' most recurring characters. If in the next Smash bros. game he made the transition from boss to a unique fighter, that would be awesome!
4. Adeline & Ribbon and/or Gooey: Although having both characters as fighters wouldn't do any harm, the goal is to have at least one character form the non-Sakurai directed Kirby games before this era. Either one would work and be unique.
I'm convinced that in order to fully represent the Kirby series in the roster, we need 1 character from the Dark Matter trilogy and 1 character from the modern series.
My choices are Adeleine&Ribbon and Magolor, because I think they are the character with the biggest gameplay potential and more representative of the 2 branches of the series, though, to be honest, the Dark Matter trilogy has Rick/Kine/Coo and Dark Matter Blade as well that are good options too.
For the modern trilogy, Magolor is the character with the biggest impact on the series; his lore, the ancients, another dimension, and his design, influenced all the characters, the stories and settings of the subsequent games.
While my personal favorite modern character is Taranza, he's like the least popular/relevant of all, so he wouldn't make much sense...
For reasons explained in my previous posts, I think that Bandana Dee is not an option, and that the only possible option for a Waddle Dee to be playable IMO is as Waddle Dee (species), with the bandanna as a costume, along with the sailor hat and stuff. But I'm sure that there are still some key characters to add before thinking of putting a Waddle Dee in the roster, especially now that the "playable regular enemy" token has been taken by Piranha Plant.
There's also Susie, but I'm not sure if she represents the whole modern series well enough... I think she's a bit too much tied with Planet Robobot specifically.
I'm not much into Gooey... His design is simple and cool, it works very well in the Kirby series, but I can't see what interesting stuff he could bring to Smash... aside of some generic moves that Kirby already does (fireball, stone, maybe copy), the whip tongue (generic neutral attack), and electric beams that Robin already does... I think he is a nice character but IMO he does not have much potential for an interesting Smash moveset.
(unless you combine him with the animals and you make a character that's like "Gooey & Animals" or something... that way it might work).
Galacta Knight could be a nice semiclone, based on Metaknight and with Link's shield, but he's a very niche character, he only appeared in the true arenas and only the most hardcore Kirby fans know him. He's by no way a bad choice, but he's very fan-service, so while I would like him, I think that he's not that necessary (unless they make him into an echo).
Then, there's Marx. Marx is an important character, and all... but do we really need more Kirby Superstar representation? Magolor and Marx play nothing alike (well, maybe black hole aside), but character-wise, they are very similar characters (both story and personality wise), so I think it's a bit redundant to have both. I prefer to have Magolor instead, because:
-he's not from Kirby Superstar, that's already overrepresented in Smash
-Magolor is the best character to represent the modern series, while the classic Kirby games are already well represented with Metaknight and Dedede
Marx is another character like Galacta Knight: I'm not against him, but he's way too much fanservice and I think he's not necessary, especially because Magolor should get the priority over him.
so, tldr:
necessary characters:
-Adeleine & Ribbon
-Magolor
Rick/Kine/Coo as assist trophies acting like in Kirby Fighters
Galacta can be an echo, maybe
Marx and/or eventually Dark Matter Blade can stay as bosses IMO
Do we need a stage based off a new Kirby game, though? Most of the major landmarks in the Kirby franchise are located in the old games. Kirby Fighters is a good example of this, almost ALL of the stages in that game come from the old Kirby games.
Try to find a stage that rivals how iconic The Fountain of Dreams, Green Greens, The Halberd, or The Great Cave Offensive are. However, I do agree that there are too many Green Greens.
The only good potential "modern Kirby" stage is Another Dimension, which has various appearances in the new Kirby games.
I'm pretty sure that Another Dimension became iconic enough to deserve some representation in Smash... but again, Mario is still missing Bowser's Castle.
Great Cave Offensive was such an unnecessary stage and really should have been Another Dimension or something
Actually, I think that Great Cave Offensive is a nice stage... maybe not for competitive play, but it's well represented, and it's set in an actual environment instead of being a floating pice of land in the sky.
We have too many Green Greens stages, and the Gameboy stage was the most unnecessary instead.
I think that with the sky platforms with a Whispy, and that steampunk thing that looks nothing like Kirby art-style wise, Great Cave Offensive is the very only stage that captured the spirit of the Kirby series correctly. If anything, we should get more levels in that style... I mean, art style, not level design.
(EDIT: I forgot the Fountain of Dreams, that's good too).