-Mega Charizard X can give up its useless 130 Special Attack to get a neat Speed tier. Not going to declare it a good one yet, because it's going to take forever to identify all the viable Speed tiers, but at the minimum it ensures that it outspeeds base 100s. Heck, it ensures it outspeeds Arceus! And those are reliable benchmarks to some extent, due to Mew and company and Arceus having "swap has no effect" statlines.
-Mega Blastoise can jolt up to 103 Speed at the cost of its Attack. Again, pulls ahead of the likes of Mew.
-Nidoking is still largely Nidoqueen's superior. :( More importantly, it can swap its 102 Attack in place of its Speed at no real cost. Or you can boost one of your Defenses, I guess.
-Clefable is restricted to minor improvements, all around.
-Ninetales can... well, it can swap its Speed for Special Attack if it wants to emphasize the wallbreaker angle, and it can swap its Special Defense and Special Attack straight-up to hold its 100 Speed. Honestly, it's nothing particularly good here.
-Wigglytuff can swap its irrelevant Attack onto either of its Defenses or its Speed for a minor boost to its viability.
-Vileplume can swap its irrelevant Attack for its Speed. 80 Speed still isn't that great prior to Chlorophyll, but eh.
-Support Parasect can almost pretend to have access to a fast Spore. 95 Speed! Doubt anybody will care, though.
-The only thing Venomoth can do of any note is improve one of its defenses at the cost of Special Attack, if you want to emphasize it as a Quiver Dance passer.
-Dugtrio can slow down and have an actually decent Attack. Note that it gets Sucker Punch!
-Primeape's Special movepool isn't so bad, and now it can cover that end of things with a simple swap.
-Defensive Arcanine is pretty directly improved in this meta! Offensive Arcanine can hit a respectable 110 Speed if it wants to run as a fast Special attacker, too, though I doubt that'll be particularly amazing.
-People have, of course, already commented on Physical Alakazam having potential. It's too bad its primary STAB is pretty sucky, though.
-Special Machamp hooray!
-Victreebel's mixed offenses can be sacrificed for more Speed -105 Speed Special attacker sounds a lot more viable- or can be used to shore up its defenses, which is nice for a Chlorophyll sweeper.
-Wall Tentacruel can give up its 100 Speed to fix up its mediocre 65 Defense. Of course, it being a fast wall is actually one of its good points...
-Golem can sacrifice its 130 Defense to get an excellent Speed tier. It's way too easy to OHKO anyway, and it has Sturdy, so the loss is minimal, and unless 130+ is a really common Speed value (Hint: not likely) it'll pay good dividends. And it has Sucker Punch to hit any faster Pokemon there might be!
-Eeeh, Rapidash has access to minor boosts.
-Slowbro gets Iron Defense. Mega Slowbro can run as a Special wall that patches it up with Iron Defense. Weird thought. Or it can run Curse, if it wants to go Physical.
-Poor Farfetch'd. :(
-Dodrio wishes it had a Special movepool. :( Its HP is too low for a wall role, too.
-Muk can finally be a solid mixed wall -105/105/100 defensive stats!- at the cost of Attack, finally get its Special Attack up to snuff, in line with its actual movepool, or hit what I'm suspecting will actually be a mediocre Speed tier of 105. Mostly I suspect the mixed wall will be cool.
-It's not just that Cloyster can murder with Ice Shard. It can swap Defense and Speed and stop caring about Shell Smash's Speed boost! Though really I imagine Cloyster will probably usually be a Focus Sash Shell Smasher with swapped Attack/Defense.
-Gengar's Physical movepool is better than you might think. Not great, but it gets stuff like Drain Punch. Overall in practice I suspect it will suffer from all these new lightning fast things outspeeding it while it can't do much of anything really cool with the meta.
-Hypno's 115 Special Defense is wasted on it. Why not 115 Speed? Or Attack, it has a decent movepool for that...
-Kingler with 115 Speed!
-Electrode with 140 Attack! If you're Exploding anyway, why not be a slow Focus Sasher? (Because Cloyster is going to be everywhere and will KO you with Icicle Spear first, but details)
-Fast Exeggutor? I mean, it's still doubly weak to Bug, but it's support movepool might be more useful than trying to straight-up attack anyway. Subseed with Harvest and 125 Speed? Or you can patch up its Special Defense, turn it into a solid Special wall.
-Marowak could add 30 Attack... or it could jolt up to 110 Speed. Depends on what wacky things happen with Speed tiers, for which one proves better.
-You'd think Hitmonlee would be able to do something cool, but its Special movepool is crap and otherwise its statline is an almost perfect fit. Just run neutral. Hitmonchan is in a similar boat, unfortunately.
-Surprise 120 Speed Exploding Weezing? I'm not sure what Weezing can really do with the meta of worth, truthfully...
-Kangaskhan actually has a pretty decent Special movepool. Probably not a worthwhile one, but it's pretty hilariously flexible.
-The nicest thing I can say about Seaking is that it never uses its Attack anyway even though Attack is its highest stat. I guess Discharge teams appreciate directly improving it...
-It's really too bad you can't swap HP, because Starmie doesn't have anything really good it can do with swapping.
-Mr. Mime's Physical movepool is actually really diverse. Swapping Special Attack for Attack might have some utility?
-Same for Jynx. It's tempting to try to patch her Defense, but I suspect it wouldn't work out.
-Tauros has a pretty hilarious Special movepool, as well.
-Gyarados and Mega Gyarados, too. Scald everything!
-Flareon can finally become the slow Special attacker it was always meant to be! Or you could make it a fast Special attacker. Whatever. 130 Speed is probably nothing to scoff at.
-Vaporeon can lower Special Attack for more Defense. It tends to be a wallin' Pokemon anyway, so that has potential.
-Omastar with 125 Speed. It's not like it actually tanks hits that well anyway.
-SNORLAX HAS ALL THE BULK. Seriously, 160/110/110 defensive stats if it wants them. Holy crap.
-Articuno can stop pretending to be a Special wall and jolt up to 125 Speed! Probably not good enough.
-Special attacker Dragonite with 134 Speed, anyone?
-Hey, Ledian can actually almost use its Physical movepool: 110 Attack! Oh and it dies to literally anything while having a mere 85 Speed, but come on, Ledian can punch things now!
-Slow Special attacker Crobat?
-Lanturn has a really awkward stat distribution, but worst case scenario it can sacrifice Special Attack or Speed to slightly improve its Defense at minimal cost. Man, why are Gen II Pokemon so awkward to stat-swap?
-Azumarill can, of course, sacrifice whichever defensive stat it likes for a decent boost to Attack. Even just running Adamant is an extra 10x2 points to Attack.
-The nicest thing I can say about Quagsire is that it can improve its Special walling ability by sacrificing its Attack. Honestly, it's going to resent being forced to either do that or run a neutral nature. Little bit worse as a Physical wall in Nature Swap.
-Other than maybe deciding to Go Really Fast, I suspect Umbreon would rather keep its wallin' stats.
-The only really noteworthy things Forretress can do are 140 Speed hazard setter like in Stat Switch, and 140 Attack Explosion like Electrode. But with a better typing, defensively, and higher statline overall.
-I'm sorry, Mega Steelix wasn't that relevant to Stat Switch with its 230 Speed, and I doubt it'll be that relevant here. I suppose it could sacrifice Attack to shore up its Special Defense, if it wanted to try its hand at being a passive wall...
-In addition to the potential as a Special attacker people have discussed, Mega Scizor can sacrifice Defense to become a 140 Speed monster with no loss to Attack. The result is a bit frail, but if its typing makes up for it...
-Shuckle: like Stat Switch, but getting to keep a defensive stat of its choice. You could try to murder things with 230 Attack-backed Stone Edges, though. Trick Room sweeper, anyone?
-Mega Heracross can swap Defense for Speed to get 115 Speed. Potentially quite mean!
-... 120 Speed Magcargo?
-Octillery gets to sacrifice an attacking stat of its choice (hint: Attack) to get 105 Speed at no cost! Now if only 105 Speed was looking like a noteworthy Speed tier...
-Skarmory as a surprise Special wall could be a thing. I guess. But really, just run Blissey.
-Mega Houndoom is like Stat Switch Mega Houndoom, but vastly worse. Stat Switch Mega Houndoom wasn't run by anyone.
-120 Speed Donphan that can't take a hit?
-Hitmontop can sacrifice some durability to jolt up to 110 Speed. Given how often it's just a little too slow to Close Combat something to death, this might be worthwhile... assuming Speed tiers aren't bizarre.
-Miltank can give up Speed to reinforce its Special Defense some. Pretty good deal.
-Blissey with 75 Defense? Oh man. Or you can drop Speed, get 55 Defense, I guess, but I'm not sure why you're assuming your Blissey wants to engage in Special attacking shenanigans.
-Oh jeez Tyranitar and Mega Tyranitar are versatile and scary.
-Mega Sceptile can stop pretending it plans to use its Attack stat to slightly bolster a defensive stat. Or it can sacrifice Special Attack and get to killing things with Leaf Blade I guess.
-Extreme Speed Belly Drum Linoone with 100 Attack doesn't necessarily miss its old Speed tier. Could be cool.
-125 Special Attack Boomburst+Heat Wave Swellow? 50 Speed is awful, mind...
-Gardevoir gets enough of a Physical movepool -Shadow Sneak, for instance- that yeah, Physical Mega Gardevoir has some potential. You do lose out on that sweet ability to bypass Substitutes, though. More quirkily, you could trade Special Attack for Speed -165 Speed is insane, and very little in the tier will be able to beat it, while 100 Special Attack is potentially acceptable if you're using Mega Gardevoir to clean out a mauled team anyway. Or you could patch its Defense and run it as a wall -it gets Wish, Will O Wisp, and a few other utility moves like Taunt. Seems a waste of a Mega Slot, though.
-Thank Arceus we're not talking Stat Switch Breloom. Even so, 130 Speed Spore is nothing to laugh at, and its Special movepool is actually adequate -it gets Vacuum Wave, for one- if it wants to run as a surprise Special attacker.
-You know, if Slaking wants it can run 150/100/160 defenses. But you'd probably rather use Snorlax if you're trying to go for a Normal-typed mixed wall.
-As pointed out by others, Ninjask swapping Attack for Speed has a lot of potential. Note that its Special movepool is garbage, so even aside from 50 Speed being too low even for a Speed Booster, Special Ninjask isn't happening.
-Well, Shedinja can go up to 90 Speed or try to utilize its Special movepool... I guess...
-Mega Sableye with crap Attack but 85 Speed. Imagine it! Almost useful!
-Mega Aggron can get 70/230/140 defenses with Filter at the cost of its offensive presence. Or you could go the Stat Switch route and have an insanely fast supportmon...
-Physical Mega Manectric? I guess?
-Well, Mega Sharpedo might as well reinforce one of its defenses by sacrificing its Special Attack...
-Wailord can do the same with Attack, and should.
-Mega Camerupt can sacrifice its Attack to get 120 Speed!
-Torkoal: 140 Speed Rapid Spinner. I guess.
-Cacturne can get 115 Speed by sacrificing its Special Attack and terrorize people! Or you could bolster one of its defenses, but I don't see why you would...
-Mega Altaria is basically directly improved (run Jolly, gain a decent base Speed), but it's also got pretty limited ability to manipulate this to its advantage.
-Seviper might as well sacrifice Special Attack for something else.
-Mega Absol might as well reinforce one of its defenses with its Special Attack. I'd recommend Defense, personally.
-Same with Mega Glalie. Or you can get its Speed up to 120, though that's probably going to be a bit low in Nature Swap.
-Relicanth with 130 Speed whamming you with Head Smash! Sure, it won't survive a hit, but since when has it done that anyway?
-FAST (MEGA) METAGROSS.
-I'm largely skipping over Ubers, but oh man I'm imagining 150 Speed Primal Groudon, having sacrificed its Special Attack to get it, and it's not pretty. Or 180 Speed Mega Rayquaza by the same logic, in AG. Yikes.
-Run Empoleon as Physical, get it 111 Speed? Maybe?
-Honchkrow probably appreciates dropping Special Attack to get some bulk. Or some Speed. 105 of whatever is OK.
-Mega Garchomp with 120 Speed.
-Fast Mega Abomasnow, whether Physical or Special, has been covered, and is neat. 132 is even higher than the semi-common 130 Speed.
-Worth commenting on Weavile entirely to point out that a fair few Pokemon can hit 130 Speed, so it's 125 Speed isn't so impressive.
-Tangrowth has excellent potential as a reasonably fast attacker (As much as 110 without compromising bulk) and even more potential as an obnoxious Regenerator mixed wall: 100/125/110 defensive stats is pretty solid.
-Hustle Togekiss has been commented on.
-Leafeon can finally be the fast Physical attacker it's been pretending it is: just drop Defense for Speed! Remember, Talonflame is banned from this, so being Grass type with garbage Defense and HP isn't as bad as it sounds.
-If Gliscor wants, it can dump some Speed to shore up its Special Defense. Slightly.
-Special Mamoswine could be interesting just for its access to Freeze Dry.
-Mega Gallade can tap most of Gardevoir's Special movepool. So that's OK.
-Dusknoir can sacrifice one of its defenses to get 135 Speed with 100 Attack. Might be worth it just to beat out the myriad 130 Speed things out there.
-Only thing worth commenting about Serperior is that it can sacrifice Speed to bolster its Special Attack. 113 is a big improvement over 75! This sounds bad, until you think of Baton Passing it some Speed.
-Emboar: Why not sacrifice your Special Attack for a stat you actually want? Or you could sacrifice Attack, either works. Too bad 123 Speed is going to be outshone by a lot.
-Samurott: 100 or 108 Speed. I guess.
-Liepard: Get some bulk, since you're Prankster anyway. Inferior to the Stat Switch version, mind...
-Zebstrika is fine with sacrificing Attack for a defensive stat. Kinda minor, overall...
-Scolipede can sacrifice Attack for some more Special bulk, if it's committed to the Baton Pass thing anyway.
-Whimsicott: Who says you were using that Speed anyway?
-Special Darmanitan!
-Alomomola loves being able to shore up its Special Defense. Its Attack is useless to it anyway.
-Special Eelektross with 115 Speed? Yes please. Or you could be Physical with 105 Speed. Either or.
-An interesting thing one could do with Haxorus is swap Attack with Speed and run Swords Dance instead of Dragon Dance. Not optimal by any stretch, but it has some potential.
-Cryogonal: Might as well sacrifice that Speed for Defense if you're going to pretend its viable anyway.
-Finally Accelgor can be the wall it was meant to be!