The camera center shouldn't be raised on Spear Pillar. It's on the ground on Battlefield and Final Destination too. I actually specifically investigated this, and I used comparisons to Battlefield as a gauge of the lower blast zone (a natural change would have put it a bit lower than BF's, but I decided to make it equal).
Summit... Well, I would never freeze it (it's frozen enough!), and making that huge chunk of ice dropthrough sounds hopelessly awkward to me. It has real height to it; how does dropping through it even work? I'll check out that version of Summit sometime, but I don't have faith in it... Summit is really very low on my priority list, as much fun as the stage is (I mean that legitimately; Summit is a blast).
Anyway, in a perfect world, here's what I would honestly do to every stage. Lokee's post might take some interest on this as he did a point by point himself. First though, a general point.
Walls aren't really broken in Balanced Brawl, including Corneria. All combos that lead over to a wall and lock are fixed (both things like DDD's chaingrab that combo to and against the wall and things like DK's cargo dthrow that set up for wall locks are fixed). The only real remaining issues are a few dtilt locks and Falco/Kirby jabs. I intend to make Kirby's rapid jab at least easier to SDI; I could easily make all of the wall locks that exist at important percentages similar. I'm not convinced it's important since these are stupidly hard to land (as G&W, I have literally never landed a dtilt lock against any opponent who doesn't completely suck; the spacing is really strict and basically requires them to cooperate). In any case, if there is any evidence at all even on Corneria any of these wall locks are practically landable in real matches, I have no problem tweaking things to nerf the wall shenanigans.
Also, consider my optimal mechanics change for ledgegrabs which I would ideally spread to water. There's some internal variable which increments on ledgegrab or water entry and is set to zero upon either being inflicted with hitstun or dying. If this variable is 7 or more, you get no invincibility when grabbing a ledge and simply fall right through water as though it were air. If it is less than 7, there is absolutely no effect. This would totally kill planking and water stalling and all easy work-arounds, but it wouldn't really impede normal gameplay. If you aren't getting hit, there's no good reason to keep grabbing the ledge or entering the water. If you are getting hit, you get refreshed, but getting hit is exactly the sort of circumstance that makes stalling unprofitable while making normal use of ledges and water more important.
Battlefield- No changes
Final Destination- Introduce turtles from Mario Bros. Seriously though, no changes, and that includes the lip below the ledges (learn to recover; this is a really basic skill... more at the bottom of the post on this point).
Delfino Plaza- No changes (has water)
Luigi's Mansion- Change global stage time slowdown here to just apply to the refresh rates on mansion and not to the moving platforms or the animations. This is a low priority change. Making the second floor non-solid is not only awkward but destroys much of what makes the stage unique; smart play easily circumvents the "techfest" situation, and Meta Knight's Mach Tornado is no longer abusive (jab locks are also gone). Ideally there would be some non-intrusive change for Olimar that wouldn't make up smash chains effective but would not matter much when he usmashes in other circumstances (easier to SDI?). I should stress the stage is fine as-is; it just requires strategy (which is a good thing!).
Mushroomy Kingdom 1-1: Perfect in Bbrawl; it only really needs everything ported to .pac format (the slowdown is likely non-trivial).
Mushroomy Kingdom 1-2: This is one of the most challenging stages to work with. Ideally hitting the blocks would cause you to slow down but break the blocks and continue moving (with no opportunity to tech), and the ceiling would probably be raised a little more than it currently is to ensure upper area uthrow gimmicks are not broken. As things stand, this stage is a real techfest that smart play can't easily get around, and keeping the basic nature of the stage in-tact has few avenues to preserve this. I wouldn't hold my breath.
Mario Circuit: This stage is fine. The karts are plenty weak, and the structure is pretty fair. It's a pretty fantastic Ness counterpick, but is it so wrong for him to have such a stage?
Rumble Falls: The "speed up" event is actually unfair to very slow characters (Ganondorf, Bowser) and moreso to very slow falling characters (mostly Zelda and Peach; the multi-jumpers have easy ways around problems). They shouldn't die to it, but they basically can't do anything but barely survive during it and have to be really precise. This event should just go. The spikes are really perfectly fine, but the first spike being weakened knockback-wise so teching isn't so life and death wouldn't hurt. An unnecessary but nice change would be removing collision from platforms until they physically appear on-screen; this would remove gimmicky uthrow kills Kirby and Meta Knight sometimes get here.
Bridge of Eldin: The bomb here would receive a very slight nerf as a part of a more sweeping change to hazards (30% to 25%). Otherwise, the stage should mostly be fine; it really needs more investigating and playtesting.
Pirate Ship: The bombs really should be less powerful (the central hit can do 55% now; that's just insane even if you really shouldn't get hit... it should be more like 25% and non-fatal), and the water changes matter here more than anywhere else. This could be a good stage!
Norfair: The lava walls and the background lava wave are totally fine. I would probably make the fire pillars fixed knockback that tend to put you off-stage (so getting hit really is perilous!) but not the occasional deathtrap they currently are. This is a really good stage people are stupid about, as an aside.
Frigate Orpheon: no changes (the lack of a ledge is fine and a part of the stage's cp flare)
Yoshi's Island (Brawl): no changes
Halberd: knockback reduction on the claw to generally be non-fatal, but otherwise this stage is fine. I think the hazards make it a better starter, not a worse one.
Pokemon Stadium 2: absolutely nothing about this stage needs to change
Spear Pillar: you can see my initial take on it here! Ideally the lasers, Dialga's jumping forward attack, and Cresselia's assorted death would do less damage and probably not kill quite so easily (though some of it should still be able to kill).
Port Town Aero Dive: I'd probably drops the damage on the cars to 15% (from 20%) and make them about as fatal as Mute City's cars from melee (not very fatal but eventually do kill; I think they're about as powerful as Mario Circuit's karts actually). The wall is fine (complaints about it strike me as whiny; you surely must know exactly when it is coming by now), and the lack of ledges is also fine and probably for the best (just slam yourself into the track to recover if you can't otherwise; I don't have problems here as tether characters).
Castle Siege: I'd really love to find a fix to Olimar's glitch on the third form (for some reason, pikmin pluck is unnaturally slow here and on the nature background for custom stages... and I think one other obscure circumstance I can't remember). Otherwise the stage is completely fine, starter grade stuff.
WarioWare Inc.: Ideally the prize system would be reworked to remove the Starman and Super Mushroom prizes. This would make the prize always the small % healing (I think it's 4%) in no item games and divided between that and having an item spawn right in front of you in games with items on. A few of the hazards could stand a minor tone-down, but it's nothing drastic. I'm not freezing this stage for sure; that just sucks the soul out of what is honestly not that far from being a fair cp.
Distant Planet: The stage is fine; people need to play on it more. Gameplay is really slow here, but I'm not sure that's a real issue.
Smashville: no changes
New Pork City: Well, uh... I'd probably need to mess with both the model and the collision data in a bit more sophisticated way than we do now. I'd like to re-arrange everything here, get rid of a few of the less interesting pieces, and have it be a big stage based around that pendulum (with the breakable platform below). Ultimate Chimera stays; that guy is awesome, and you really deserve it if you ever actually get hit. It would need to be adjusted carefully to somehow keep the full background in view while simultaneously making it not quite so big blastzone wise; it's a perfect recreation of New Pork City from Mother 3 that is not worth losing. This would be a massive project.
Summit: Discussed elsewhere. A clean fix to the loop is non-obvious.
Skyworld: I've gotten into it thoroughly elsewhere, but I think the solution entirely lies in shifting the health of the breakable platforms. Ones with ledges should be harder to break, and the upper two should be easier. The remaining three can remain the same; they're fine. The ceiling situation is familiar to Luigi's Mansion; good players can and should be able to work around it through intelligent play and not just rushing at each other like scrubs.
75m: Uh... I honestly don't know where to start. Any change that moves toward fixing it moves away from what the stage is all about.
Mario Bros: If I were going to seriously try, I'd probably make all the platforms pass-through and greatly nerf the hazards (but not remove them; they're important to the stage). I'd have to play around with it to see how it played out; I can't guess easily.
Flat Zone 2: Lion tamers definitely need a tone-down, and a slight decrease to the oil panic people is maybe in the cards. The stage structurally is totally fine; this isn't nearly as bad of a stage as some people think it is.
PictoChat: Specifically I'd like to have a cleaner implementation on slightly slower drawing (so players can react better). I'd also weaken the following specific hazards: side spikes (20% -> 15%), arrows (knockback reduction), cart (knockback reduction), bombs (25% -> 20%, knockback tweak to be non-fatal even for people who suck at DI). The stage is already a really good stage though.
Hanenbow: My strategy is to see how removing the upper-most plant could work out. I'm not sure how clean interaction with the music-things can be maintained; it will need investigation.
Shadow Moses Island: The walls need to break a lot easier. They should still be harder to break than other breakable stage objects, but seriously, Sonic, Meta Knight, and a few others are just horrible here because breaking the walls isn't really very practical so characters who can't kill up are just terribad.
Green Hill Zone: As much as I love checkpoint gimmicks, it having fixed knockback would be for the best. It should be an annoyance and spatial control, not a real threat.
Temple: Well, my Bbrawl v1 wasn't exactly popular. I'm not convinced shrinking the stage even further does much good either. Careful thought is my current strategy.
Yoshi's Island (Melee): No changes, the stage is fine, learn to play, etc. Seriously though, I've had a lot of good matches here, and there is no real abuse possible. It's a decently strong cp that requires some good stage knowledge, but that is not a flaw in the stage.
Jungle Japes: No changes other than the token way it gets swept up in water business. I love this stage.
Onett: Onett's cars going from 30% to 20% would be a part of this long list of hazard business (I have an old list from as far back as last May; I've thought this out for a long time). It's not important though, and this stage is fine.
Corneria: Stage is fine.
Rainbow Cruise: Stage is fine.
Green Greens: I'd like a cleaner implementation on the slowdown so it only applies to the block refresh rate and not to everything (slow blowing Whispy Woods isn't exactly the greatest side-effect). Bomb blocks and exploding apples should do 10% (instead of 20%) and have less knockback but should both remain in their current probability. A fix to the exploding field glitch would be nice, but the stage is fine either way. A fix to the assorted minor bugginess on this stage (Olimar's pikmin of only certain colors sticking to blocks, disappearing transforming characters, upside-down wall clings) would be doubly nice but a total luxury.
Big Blue: The current version seems fine to me, but it needs more play testing.
Brinstar: The stage is fine.
Pokemon Stadium 1: The stage is fine. The windmill is changing over my dead body.
Anyway, funny story about "bad ledges". Back when melee was the game of the day, I never understood what people were talking about with Battlefield's "bad ledges". It really and legitmately confused me, and I ended up asking someone who took the game more seriously. It took something like half a smashfest with me just not understanding what he was talking about, and then I had a match using Mr. Game & Watch where I mis-aimed my up special and got caught under the lip. He said that was exactly the issue, and at that point I finally understood what people were expecting. For some reason, some people were thinking that sticking to the edge of the stage should let you easily slide up to the ledge. I still haven't quite figured out why that's the case; I always looked at getting caught under a lip as about the same as making any other technical error recovering. Maybe it came from maining Ness for so long. I aim my recovery wrong, and I die. Having to aim at the ledge just kinda makes sense to me. Actually, the fact that a lot of characters could utilize sliding up the side of some stages (like melee FD) was a bit of a surprise to me, but either way, I can't see any problem at all with ledges like Brawl FD's. Just... aim at the ledge. It's not an issue for any character.
If someone can really explain to me why sliding along the side of the stage to reach the ledge should be rewarded or why it's actually important for some characters (as opposed to just aiming at the ledge in the first place), I would rethink for FD and a few other stages, but I'm skeptical such is possible. I really and truly just can't see a problem here.
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A quick bit about DK's giant punch. The reason it's not fair to compare to bair is that bair doesn't kill at almost any percentage off-stage. DK's giant punch also has super armor; your odds of being able to "avoid it on reaction" are slim to none. I mean, if Giant Punch only hit as hard as bair, it would be fine to not have helpless, but it hits more like fsmash than bair...