Something about Piranha Plant, Bandana Dee and generic enemies:
Piranha Plant will probably be the only case of generic enemy to fighter. I think that the plant is not intended to be a regular fighter, it's a bonus character, special content for people who preorder the game.
It doesn't appear in the mural, it doesn't appear on the box (assuming that the infamous box from the box theory is the final design), it's just there.
Probably they have chosen a random and unimportant character so that people who buy the game late will still get all the major characters and experience the full game, even without the little bonus that's Piranha Plant.
It's also an unexpected enemy. It's not a Goomba or a Koopa, it's purposely an enemy that makes absolutely no sense to be a fighter, it's that because they wanted to surprise people, it's the real WTF character.
So, I don't think the roster is open to generic enemies yet, IMO Piranha Plant is just a bonus and not the beginning of a new trend. It could have been Sandbag for what we know.
The "problem" is, while the character is absurd, the amount of effort put into it is incredible, the moveset looks very fun and creative, all the moves are very faithful to the source material, it's one of the best crafted characters so far! (but there are a couple of things that bug me: where's the fireball? And, why Petey Piranha has those cages when it's not canon and it was just a Brawl thing? After they changed Giga Bowser to his canon design, I expected other characters to get the same treatment). So, it deserves or not, I don't care anymore, the character seems very fun, the moveset is cool, and I accepted it because of this.
Anyway, I'm not against generic enemies as long as they are iconic and help representing the franchise in a better way.
If I keep thinking, I can find other enemies who could become nice fighters... for example, an Omega Metroid from the Metroid Series (maybe it's a little obscure as a form, but a metroid itself in the roster would make sense given the name of the series, and I know that Dark Samus is one, but she looks nothing like a Metroid).
I mentioned Chef Kawasaki... Chef Kawasaki appears a lot in Kirby games (even though it's mostly in minigames), and usually his appearance is treated differently from those of the other enemies... he even has a theme song! (from Gourmet Race, a minigame where he doesn't even appear but it's still associated to him due to being about food). And, in Japan, they opened resturants (Kirby Cafe) where Chef Kawasaki is a mascotte, and being the chef, it's even more central than Kirby and the Waddle Dees. Chef Kawasaki IMO is something between a main character and a regular enemy, he's a mix of both.
Kamek and Skull Kid are also deserving for their role in their respective games, though I feel like they miss some moveset potential, and I think Kamek works better as a support for other characters than as a fighter himself (in the source games he's often seen powering up enemies and running away, even if there are some bossfights, I think that he being a support for other characters is more iconic to him).
Now about Bandana Dee: my problem with Waddle Dee is not that he's a generic enemy, my problem is that I think he's too generic as a fighter, and it's not because he's a generic enemy, it's more because his signature weapon is just a regular, basic spear, and all what he does is stab with it, in a very basic way (ok, he also used a parasol, but it's still a basic stick-shaped object too, can't do much more than slashes and stabs). Look at how creative Piranha Plant's moveset is...
This is only partially a problem; in the roster, there are already some fighters with generic movesets based on simple and basic actions; such as Little Mac, and the Marths. The problem is, the Kirby series features several characters with unique skills and great moveset potential; Bandana Dee is IMO the least interesting among them, and getting him instead of the other characters is IMO a wasted opportunity for a crazy unique moveset.
Another problem with Bandana Waddle Dee is that he's only one specific iteration of Waddle Dee, and that's limiting IMO.
Look at Piranha Plant once again. It's not one specific version of the plant made into a character, it's the amalgamation of all the iterations of the plant made into one, and it's still called Piranha Plant as the whole species, because it represents all of them (it's not, like, "Fighter Piranha Plant" or something).
I think that if Waddle Dee is ever going to join the roster, it shouldn't be Bandana Waddle Dee, but Waddle Dee in general. Waddle Dees have done countless actions in Kirby games that go way beyond the use of a spear or a parasol. Using Bandana Waddle Dee IMO is just a limitation, especially when you can still represent him as a costume, and also represent Sailor Waddle Dee, Kirby Cafe Waddle Dee, etc.
Also, just like Goomba and several other enemies (Motobug, etc.) Waddle Dee is seen as weak, helpless (excluding exceptions that are obviously just exceptions and not the iconic version of the character).
This is why I once suggested that Waddle Dee as a character whould be a group of Waddle Dees instead of just one.
In a different thread I mentioned a concept, "Waddle Dee Crew", that would consist in a mix between Ice Climbers and Olimar, where you control the leader Waddle Dee (with an hat) and you are followed by 3 more regular Waddle Dees who help you; the moveset would feature the Leader using spears, parasols and other stuff, and the rest of the crew doing some team moves with the leader (mostly specials).
(additional note, I thought of this concept before the Grinch leak existed, this came before the idea of Chorus Kids + Conductor from that "leak", even though it's extremely similar).
So, I'm not against Waddle Dee/s if done right, it's just the concept of Bandana Waddle Dee that people want that I think is wrong, and Piranha Plant (being based on the species in general instead of being a specific iteration of the plant) proves it.
This said, there are countless Kirby characters who I want to see before Waddle Dee, and I'm against the inclusion of Waddle Dee in Smash simply because there are more important and unique characters yet to add before him. Piranha Plant is in Smash, but the Mario series is already well represented, excluding Waluigi and Pauline (who anyway appear in the game in different forms) the Mario series is pretty much complete, only obscure characters and spin-off characters are remaining; it makes sense to add iconic enemies since they have finished the actual characters. Kirby is far from being represented that well (and I fear it will never be as long as Sakurai stays as the director).
Put Adeleine & Ribbon, Marx, Magolor, Chef Kawasaki and Dark Matter in the roster, then we can start discussing about Waddle Dee.