Are the Original 12's gameplay designs as outdated as people claim to be?
My opinion on the original 12:
I don't know
Ok
Maybe a light revamp
Completely revamped
-Mario
Imo part of his moveset is fine, but some moves, special mostly, would be better if revamped a bit.
Most of his neutral game is based on his gameplay in Mario 64; while that's fine, it's also true that in the most recent games, Mario does not punch and kicks as much as he did in 64, he fights more with ground pounds, spins, rolls, in a more cartoony way. Anyway, this is not much necessary as a change, and it would definitely be harder to adapt into a fighting game than actual punches and kicks, so it's ok maybe.
The most outdated part of the characters comes from the representation of power ups. The F.L.U.D.D. is a nice gimmick from one single game, but it's not a defining trait of the character... it's not as much iconic as the animal suits that debuted from SMB3 and in recent times influenced the design of most of the newer power ups such as cat mario and others.
Overall, Mario is kinda fine, though some revamp would not be a bad thing to do.
-Link
This is weird. He got changed a bit to represent BotW more, but in this process, they made him lose some representation of the rest of the series, since remote controlled bombs is something a bit too much specific to only that game. Luckily, Young and Toon still do the job of representing the "generic Link" more. Anyway, Link's moveset, assuming we're talking about the Smash 4 one, so a more generic "Zelda series" moveset, is mostly fine, but this does not exclude the possibility of revamping him as well. Considering how puzzle oriented the series is, he could take advantage of some puzzle element as well, such as some moves based on magic spells/items, maybe even just something as simple as the ability to switch elements to the arrows (thunder, fire, ice, and maybe a mana gauge that you can run out of, making the arrows lose the element completely). Playing magical instruments is also one of the most iconic things of the series, and it's weird that the only reference it got in smash was in the opening sequence of Smash 64 and Melee; I would put it somewhere in the moveset, even if it's just a taunt. mechanically speaking, he's fine, the only thing that maybe I would revamp is the hookshot... maybe let the player aim it freely or something...
-Pikachu
I'm not a fan of Pokèmon so I'm not sure... from the prespective of a not-fan, Pikachu is just Pikachu. I would say it's fine but I don't know, really.
-Kirby
He's not outdated, but I think that his moveset is a bit limited regardless. I know that the characters hats is a beloved feature, but I sometimes think that they should develop a set of standard abilities (like in the original series, Cutter, Hammer, Sword, etc.) and let him gain one of those standard abilities from characters instead of getting their face's hat (For example, Link gives him the Sword ability, Peach Parasol, etc.). I know, they're funny and all, but it's not a faithful representation of the series, Kirby in most of the cases does not copy the look of the enemies he eats. Speaking of ability representation, well... stone, hammer, cutter, a bit of yoyo... there are a few abilities in his moveset but I think that with the huge amount of abilities he had in it's original series, that's still too small of a "roster". So, instead of representing 4 or 5 abilities, I would remove them completely (unless he inhales a character) and make his base moveset more basic. In other words, I would redesign Kirby completely.
Up special absolutely has to be a Warp Star (I don't care if it's an item already, it's just too much iconic for Kirby that it has to be in the moveset, and it makes a lot of sense), while other specials can be inspired by other elements of the series, including spin-offs: in Canvas/Rainbow Curse, Kirby has shown to have some natural "Ball Ability" skills even without copying the ability, so a few moves can be based on rolling and bouncing like that; in some of the older games, Kirby has the ability of multiplying himself up to 4 when he does the Kirby dance... maybe use that effect for some spectacle, such as, a Smash attack is 4 Kirby launching themselves into the other player and then 3 of them just disappear and only the main one remains. Maybe give him the ability to create star blocks from shooting stars, using Star Stacker as source for the move; this is not just for reference, but also for playstyle: a block can be inhaled and launched at the opponents as a projectile; also give him freaking air puffs as attack. Overall, he needs more stars, he's Kirby of the Stars (Hoshi no Kirby) and stars are barely represented in his moveset. Inhale and spit need to be more central in his moveset.
-Samus
Samus is still the old static and tank-like character as she was depicted in the older games. Starting from Other M, then Samus' Return and now Dread, she has been depicted more as a fast and agile warrior who's capable of fighting both melee and with ranged attacks; she has gained a wide variety of new powers and she's now basically a completely different character. The focus on mobility and speed makes the existence of Zero Suit (light and fast counterpart) unnecessary if you ask me.
-Donkey Kong
Donkey Kong is kinda fine I suppose... the only element that's missing in his moveset is barrels, and at best I would change it as his new up-b. But that's it... maybe you can make him summon normal barrels somehow too and launch them at opponents? But you would need to replace an existing move for that, I'm not sure if it's a good idea. Those small changes would change the playstyle of the character a lot though, making him more agile (new up-b is being launched by a cannon) and have more range (rolling barrles). He would obviously need to be rebalanced to accomodate that.
-Fox
Fox is ok IMO, and probably the best made up from scratch moveset that represents the gameplay of the original series despite being completely different from a fighting game.
-Yoshi
Yoshi IMO can be revamped. Now the hardware can handle Yoshi being followed by a line of eggs like in the original game, and maybe them being buffed to travel in a straight line and bounce on walls. You would miss the gravity and the mechanic of aiming the eggs by holding the button for the right amount of time and all, but you can still move that part of the moveset onto another more fitting character (such I don't know, Worms?)
-Luigi
When Luigi was introduced in Smash, he was Player 2 and weird Mario's brother. Now Luigi is a way more defined character, different from his brother enough to deserve a completely unique moveset that shares nothing with him. I would completely change the character to keep the Luigi's Mansion series as the main focus.
-Jigglypuff
Same as what I said for Pikachu; Honestly Jigglypuff is the only one of the original 12 who I feel like could be cut as I don't think it's that much necessary.
-Cpt Falcon
He's ok, one of the most iconic Smash characters, fine as he is. Aside of balancing, tuning and refining, I would keep him as he is.
-Ness
Never played the source game but I think he's also fine as he is. Since most RPG characters have complex gimmicks (menues, mana gauges, stuff) maybe he can get some of that stuff too but since I didn't play the original game I don't know really.