Bowser, Charizard, Chrom, Cloud, DK, Little Mac, Lucina, Marth, Meta Knight, Mewtwo, Ridley, Roy, and kind of Pichu, Pikachu, the Pits, Sheik, and Yoshi would have been better examples for your fast-moving characters with "wide hitboxes and linear knockback."
Except for Utilt, Up Smash, and Side Smash, all of Fox's attacks are linear and don't cover a lot of range. Part of that is because he's small, he's the same height as (retconned) Little Mac, and the other part is how he fights as an unarmed martial artist using primarily straight-hitting kicks. Some of his moves have high active frames like dash attack, Utilt, Nair, Fair, and Dair, but they're also mainly close his body. Uair is the the most extreme example since Fox does a frontflip facing the screen or the background. Falco actually the does the same move, more noticeable in Brawl and Smash 4, but he faces his opponent, so the hitbox arcs above him rather than hitting only above him. And yet despite the hitbox, Fox players have been able to juggle with Uair very well to the point where Fox's Uair is a staple move. I'd say the main reason for why Fox's Nair doesn't miss much is because it's a static hitbox that has high active frames giving you so much time to hit with it as much as its hitbox is fairly generous like other "sex kick" Nairs. That, and Fox can constantly spam it, so even if he does miss with it, he can try again fairly quickly.
Fox is a high precision character given his hitboxes and that he's a glass cannon who cannot afford to get hit as much as other characters due to his low weight and high fall speed and gravity making it easy to combo him. At the same time, Fox also happens to be a very fast-moving character with fast attacks.
I was using Fox as an easy example, since everyone generally understands how he plays. His kit is designed for speed and is linear, because it would be difficult to handle his level of speed, if you had to worry about varying knockback angles and DI. His moves are precise, which is why he has dash attack, Nair, and Fair to help combo into them. They have generous hitboxs, that pop the opponent up in a certain way, which will lead to juggles with Up-air. Zelda does not have similar moves, and having more speed might be detrimental in that regard, since her moves are precision oriented, without a whole lot to combo into them.
People are already having difficulty landing sweet-spots or properly linking Nair, I felt adding more speed into the mix, would exacerbate that.
I'm not sure what would make her so different from Marth or Roy if she were faster moving. Or Fox, Greninja, Little Mac, Meta Knight, Sheik, and ZSS to name some other characters who move very fast, but also require precision or can be played very precisely. Ignoring Melee Marth and Roy who had their own quirks in that game and Roy not being that fast-moving in Melee, Marth and Roy players constantly have to deal with their sweetspot and sourspot mechanic and in each game, Marth has been moving faster and faster while Roy jumped up from average in Melee to a speed demon in Smash 4. Roy's power lies in hitboxes closest to the Binding Blade's hilt, but Roy also gets a lot from the tipper sourspots. Granted, they use swords, so their moves are safer, but the point still stands where in spite of their movement speed, their players have been able to space and choose their hitboxes.
They have high amounts of mobility, quick frame data, disjoints, and are overall fairly safe on whiff. They can whiff or mess up an input and almost immediately have another buffered. Some of them, have wide and generous hitboxs, helping with properly spacing. The trade off most of them have, is difficulty KOing, whether due to lack of power or KO options being difficult to land. Roy and Marth in particular, are considered vastly inferior to their Echo counterparts, because of the difficulty and risk, in spacing their sweet-spots. They aren't that great in this game overall, compared to similar characters.
Zelda does not have the safety, mobility, or generous hitboxs, they do. Whiffing means a free hit a lot of the time and being floaty and easily comboed, means she will suffer much more. You can buff her mobility, hitboxs, and frame data to compensate, but you will run into the problem of her KO options being stupidly powerful. That will create an imbalance, with her neutral and safety being good and her KO options being much easier to land.
People are also not asking for Zelda to move stupidly fast. If Zelda ran as fast as Sonic, then I could see players having some trouble, but that kind of dumb speed might benefit her more than hurt her. The movement speed buffs people are asking seem more like quality of life requests. For example, the air acceleration thing. Zelda can still move as slow as she wants if the base value isn't changed, but she can also reach her top air speed faster if the max additional value were higher. You get about the same degree of control as you would now, but in times when you want her to move as fast as possible while in the air, she'd be able to do so quicker.
That air speed buff, is perfectly fine and I've always felt it could use a touch up. I just never mentioned it, because it wasn't really a deal breaker at the same time. I was more responding to run speed and ground speed buffs. I don't think she will benefit from any speed buffs too much, with what her kit currently is.
For every argument that Dtilt or NAir leading into her own elevator or kicks and that being bad I just have to say: if Greninja can chain many falling UAirs into almost any move, why is Zelda’s suddenly worse when she’s already overall got worse stats than the frog? If Peach can have a Dtilt that leads into 50% or many characters have Dtilts that lead into potent moves, or HAVE YOU SEEN Ike? It almost feels like half his moveset can string into an UAir KO.
Greninja's opponent kind of needs to be in disadvantage, for that to really work well, Peach is widely complained about and even top players, are bullying Samsora for playing an "easy character", so I don't think that's a particularly good example. Ike, as has already been mentioned, heavily relies on Nair and when Parrying becomes more refined, I expect to see him drop significantly, because that Nair is Parry food. All of those(well don't know about Peach) require the opponent to be at a high percent, to actually kill.
Zelda in contrast, could do it at a low percent and in neutral, which is what makes it a little too strong.
These kinds of things are natural in this game with so many chars, and help strengthen their viability. Zelda had these too, but it feels like both of these move qualities were specifically targeted and snuffed out for Zelda when it c l e a r l y wasn’t because the devs wanted them removed as a design as a whole, just for zelda.
I'm fairly certain nearly all Nair, throw, and Tilt combos, got the same treatment, didn't they? Most of the other Nairs I've used, behaved the same way and a lot of the whining pre-release, was over returning characters not having their combos from 4. I'm fairly certain the goal in Ultimate, was to move away from all of those combos and Ding Dong like things in general. I think the issue was characters that weren't designated combo characters, had all these combos and kill confirms. Smash 4 was basically Down-Throw the game.
If Zelda could Dtilt to LK it would’ve been perfectly fair in her current state, so would falling NAir to elevator. “She was trash then but not now” doesn’t make that a valid reason to remove them when she’s universally agreed to not be close to the top of the chain, and removing them kind of works against the reasoning that she isn’t ‘trash’ now. Honestly I’m kind of appalled that some people can say she could make high tier right now with so many poorly functioning designs. We’re all beating a dead horse with these two moves but Zelda was done dirty, in kind of a way no one else was. It’s just sketchy that some of her competitively viable strategies were cut down to limit her options.
Her not being the best, isn't really a valid reason to keep any overpowered options either. It would only work against that reasoning, if you believed those were her only options. Zelda can potentially KO from anywhere, with almost anything and at lower percents than is the norm, even for heavies. That's why giving her safe setups, into her most powerful KO options, isn't really a good thing balance wise. She would basically become Smash 4 ZSS, without the precision and retaining the ability to KO at high percents easily. I can't think of any character in the game, ZSS included, that could do the same, at the same percents, as safely or easily.
Now,
If it was really intentional design to get rid of these specifically “broken” things on Zelda then we can expect them to never return. At that point it’s time to find other moves that could be tweaked to create solid and new “broken” options. (Broken being used loosely) is personally opt to have a strong Lightning Kick set or generally solid aerials. That has been a long term short coming for her and it wouldn’t be hard to make the moves impressive. Honestly they should do a thing or two that could come off overpowered because if you actually analyze the potential of even a 1F long but large sweetspot on this move it still only just comes to par in usefulness with other powerful side aerials. Ganon BAir says hi.
I find using her current moves, in conjunction with Ultimate's new mechanics and flow, and pairing her Up-B and Fair/Bair, with punishes and reads, works just fine. Rather than tweaking moves, I think it would be more productive to tweak strategies. We can't really affect how characters are going to be changed, so we should just work with what we have. Sakurai seems pretty confident he got it right this time, so I don't think we should be expecting too much in the future.
I think she still has pretty good stuff, like Dair->Up-B at mid percents. That has a sour-spot, so carries risk and wouldn't be broken like comboing from Nair and D-Tilt. I mean, just remember how much whining there was pre-release, that returning characters didn't retain their combos and strategies from 4. Literal Fox, was considered low tier and potentially the worst character in the game, even by Larry Lurr and other Fox mains, simply because Side-B didn't cross people up anymore. People eventually discovered how their characters were supposed to play and devised new strategies and game plans around it. I think we need to do the same, because I highly doubt the old stuff is coming back.
Falling Nair for Zelda I don't think is that much to ask for. Other characters have falling aerial confirms for hits or kills and theirs are just as powerful or even more powerful than Zelda's options.
That being?
Fox has been doing essentially falling Nair to Up Smash for years and Ken's Nair's autocancel window was improved in 2.0.0 and with the change to Ryu being able to special cancel with aerials in Ultimate and Ken's Nair being a kick instead of a short-ranged punch means Ken's probably going to be doing more stupid stuff with it now. Granted, the difference is that Zelda's Nair could drag people down, but she still needs to be able to hit with it and Zelda's not a fast-moving character on the ground at least and Nair's hitbox isn't that large.
Those aren't easy to do, disjointed, and as far as I know, still wouldn't KO as early as Zelda's hypothetical combo. Zelda's drag down Nair, was pretty easy to land and there wasn't much opponents could do to avoid it, given it was from a neutral tool. Zelda's neutral has been buffed, so it will be much easier to land, which is where the unfairness lies, since Zelda isn't as bad as before.
Zelda could literally do all those options with her Melee LKs, they could be spaced, combo into each other, sourspot combos into sweetspot, could be SH AC and did good damage. With Ultimate mechanics these would be a having a sword for a foot. By giving us the AC window Zelda's kicks would make up for not only being the same move, but would further increase her lack of neutral. Possibly even her disadvantage state. The only reason it didn't completely save her was because the Melee engine did her no favors along with having a laughably bad design. I'm no expert, in fact I'm still trying to learn the game, but even I dislike how her kicks changed from being consistent to requiring precision. Of course this is just my opinion and not saying you're wrong in any way.
Sure, but at the same time, Melee was full of broken BS, like a Down-B that could KO at 40, a Dsmash that could do 60, an infinite chain grab, frame 1 Shine spiking, and all sorts of others. I hardly think it would be fair to include something from Melee, where balance was everybody having something unfair by other Smash game standards. You have to understand that having one of the most powerful KO moves in the game, being a single digit frame aerial, is already pretty good. Even equivalent moves like Ganon's Fsmash, forces him to be in place. You can do all sorts of things with Zelda's Fair, that a lot of other similarly powered moves can't. That's why there needs to be that trade off. You can't very well have the usual drawbacks like lots of startup or endlag, on an aerial, not with how they did aerials in this game.