Re: Protean
Before I begin please note I am not trying to argue for a ban, I'm personally on the fence, but I think people are misunderstanding what I think the potential problem(s) with Protean actually is/are. I don't like people dismissing my concern on an inaccurate basis.
Point the first: Protean reduces a Pokemon down to its stats alone for determining usability. This is arguably true of a lot of stall donors, but stallmons don't punch holes in teams in one turn. This makes Protean uniquely capable of being slapped, viably, onto virtually any competent offensive choice. And you can still slap it onto stall choices if you want to take Kecleon's access to Recover and Stealth Rock as a starting point, so just assuming Protean=offense is not a certainty!
Contrast this with other offensive donors, and you will find they all struggle with some degree of limitations. Keldeo, for instance, is a fairly limited donor, because Lucario is basically the only other Specially-strong Fighting type to take advantage of STAB Secret Sword, and outside Secret Sword Keldeo's movepool is limited and overall inferior to myriad other choices. (And Justified synergizes poorly with inheriting from Keldeo for its Special movepool) Talonflame is a relatively predictable donor, because if it's not a Physical Flying type, Talonflame is an iffy donor for it. I can go on forever here -Guts is bad on Fire types and useless for Special attackers, Toxic Boost is useless to Poison and Steel types and Special attackers, Flare Boost is useless to Fire types and Physical attackers, Mega Launcher is useless to Physical attackers....
Point the second: Arguably a sub-point of the first point, Protean can take any bad or problematic typing and make it go away. There are narrow, specific cases of other Abilities allowing one to enhance a specific typing (Desolate Land covering for a double Water weakness, Primordial Sea or Flashfire covering for a double Fire weakness, most notably leaving Bug/Steel with no weaknesses), but these can all be predicted to an extent and in some cases actively announce themselves on switch-in. (eg Primordial Sea, potentially Levitate if you have Spikes and/or Toxic Spikes and/or Sticky Web up) Only Protean can take literally any typing and make it better.
Point the third: Within battle Protean provides benefits that overlap with the utility of U-Turn/Volt Switch/Baton Pass/Parting Shot -if you outspeed the target (Or use priority!) you can predict their attack, get in damage, and go from being vulnerable to being resistant, or even immune! (This being equivalent to U-Turning out into something resistant or immune to the oncoming attack, albeit lacking the part where you see the enemy's choice to switch or lack thereof first) This provides Protean users with remarkable durability, effectively doubling or quadrupling (Or more) their relative durability against what should be able to do severe damage. Hi Fur Coat, I'm laughing at you right now. Anybody who went through all my replays might have noticed I started out with Life Orbs and transitioned to Leftovers, and my team's survivability went way up once it wasn't killing itself.
You'd think this would be a trivial detail against stallmons, except even then it can be huge. Flamethrower or Fire Punch allows Kecleon inheritors to spontaneously ignore Will O Wisp, Toxic Spikes (Let alone Toxic itself) can get in work while laughing at a Toxic attempt, Grass Knot can provide Spore immunity (Which is not trivial given how often Spore inheritors aren't Grass types -Terrakion is a favorite, and Grass Knot is excellent against it anyway!), and of course Thunderbolt/Punch can block a Thunder Wave attempt. Or heck, you can Thunder Wave them first!
It can be ridiculously hard to get in reasonable damage on a Protean target, allowing even relatively fragile inheritors to just keep going and going and going...
Point the fourth: Protean is not centered on Mega abuse. This might seem a bit odd to point out, but it's true -Boomburst is mostly notable if donated to Mega Gardevoir, Mega Altaria, or Mega Glalie. Extreme Speed is mostly notable if donated to Mega Altaria or Mega Glalie. (Or Adaptability Extreme Speed can be granted to a Normal type via Mega Lucario, but you still have some clue) Several of the really extreme beneficiaries are inherently restricted by the fact that they have to be a Mega to really produce the most absolutely ridiculous results, making it easier to predict and account for the possibility.
Point the fifth: Protean doesn't announce itself. While this is true of most possible donors, some of the most alarming ones provide an immediate announcement of the possibility upon switch-in -GeoKiss is not only an obvious possibility at team preview, but is explicitly confirmed the instant it switches in and Togekiss has a Fairy Aura. Rain-boosted Water Spout shenanigans are revealed the instant Primordial Sea kicks in, and still extremely likely if ordinary Drizzle is what kicks in. Rayquaza's powerful movepool (Extreme Speed, Dragon Ascent, V-Create...) is bluntly confirmed by either Air Lock or more typically Delta Stream kicking in. Most of the most dangerous things that need to be responded to immediately give away the game the instant they switch in.
Point the sixth: Even though Protean announces itself upon using a move, it remains unpredictable. Even once you've narrowed it down to Greninja or Kecleon, it's still really hard to say what the rest of its movepool looks like, and it really matters that you don't know. Depending on their movepool, maybe X switch-in will flat-out stop them, or maybe it be hit with a super effective Knock Off on the way in. (Or maybe they'll just U-Turn out, ha ha) Other donors, once identified, can be predicted to a meaningful extent and accounted for -especially since anything in their movepool that isn't a STAB move will be weaker and thus even a switch-in vulnerable to the move has decent odds of being able to switch in on the move. (Assuming it's not a double weakness, at least) A lot of donors, even ones with wide movepools, will still in practice tend to amount to a very restricted set of possibilities for the Pokemon inheriting, because anything else would be so sub-optimal that it only makes sense as a lure, if that. This doesn't remotely apply to Protean, where even not knowing the fourth move can be cause for nail-biting and bad decisions -one of my matches my opponent was legitimately concerned my Pokemon's last move was Sucker Punch, and this nearly cost them the match because they didn't want to risk losing their Fake Outer to a Sucker Punch. (It was Knock Off)
Point the seventh: For any move that wouldn't normally be a STAB move on the Pokemon, Protean is a Choice-level boost, for free. It's easy to say "well Protean is less of a boost for STAB moves than Guts or the like", but that's misleading. Dragon is a bad offensive type in Gen VI, only really worthwhile to run on Dragons because it is a STAB move -and Protean Haxorus cares not one whit that it's abandoning Dragon STAB because now everything else (ie the moves with actually good types) has a Band-level boost off its 147 Attack, with no disadvantage. So too for Normal attackers, only more so, also Psychic is a depressing offensive type, and even examples like Bug and Grass that are heavily resisted but technically have offensive value can get use out of it too.
Meanwhile all the Abilities that substantially upgrade a Pokemon's broad offensive prowess fall under one of two categories.
1: Banned. (Huge Power, Pure Power)
2: Either the boost is its only advantage (eg Tough Claws, which is a weaker boost anyway) or it has an inherent disadvantage to using it (Guts, Flare Boost, Toxic Boost) or both. (Hustle)
Nothing else legal has anywhere near the level of offensive pressure without either having a direct cost (Health and your item slot for Guts) or being so narrowly focused it makes you very predictable. (Strong Jaw and Mega Launcher) And then they still lack the other qualities that make Protean amazing! (Albeit some of them have their own advantages)
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tl;dr version Protean is not just flexible/versatile, otherwise I wouldn't care because that's pretty intrinsic to Inheritance, it has other qualities people seem to be ignoring that are completely unique to it and actually legitimately amazing.