My turn:
Landorus-Therian
I think the biggest problem with this is that too many people are focusing on countering only lando T. There are several mons that have the ability to handle most if not all of the sets it runs. Regirock, intimidate manaphy/suicune, rhydon, rhyperior, zapdos (if rocks are up and zapdos isn't poison heal/intimdate then it has a 25% chance to 2hko after lefties), flash fire skarmory, etc. As for protean lure sets: lure sets are and always will be a thing in pokemon. Having lure sets does not push it into overpoweredness, even OU scarf lando has HP ice sets to hit gliscor/other landos. Having a lure set just means it's a standard threat. Anything that is common and prepared for can run a lure set to beat common switchins, that's just good pokemon. I think lando is a metagame defining threat, but I do not believe it to be broken. Yes it does warp the metagame to an extent, but even talonflame does that in OU and it's not remotely up for a ban. Similarly having U-Turn to go into trappers is not unique to landorus, and does not contribute to banworthiness, especially in a tier where literally everything has access to u-turn/volt switch.
There are several problems with lant that make things that much worse. The first of which is simple, its easy to abuse and fits on about every team imaginable, while it
forces every team to dedicate a teamslot solely to counter it. Teams without Lant counters are basically 6-0'd by it (besides mega glalie teams or those with multiple sources of priority), and the mons used to counter it are limited to defensive roles hard to use on offensive playstyles, which need a boon. Many possible counters are also easily beaten outside of lando, with the exception of maybe suicune/manaphy, since they are not all that useful outside of countering lant. Not only that, but most lant
should be running wisp, as adrian mentioned, which cripples each of the 3 rock type counters you mentioned.
You mention how lure sets are inherent to a mon, but think of it this way: The lure sets are just as good as the regular ones. This is actually not something that is normal with lure sets. The main reason lure sets aren't justifyable to make pokemon overpowered is because of the opportunity cost of using them; since they are the only one with said niche or lose the ability to hit certain mons by running such a set. The problem with Lant is the opportunity cost is literally zero since the role is easily fulfilled by another mon - say, archeops. Its probably to the point where lure sets are more viable, because flipping the roles of two common mons such as lant and archeops provides no detriment to your team while luring out potential checks and counters of the opponents team and killing them. If I run protean Lant and lurekill the opposing stall teams suicune, all of the sudden my GW Archeops sweeps easily. Why is this a problem?
There is zero opportunity cost in running a different lant set. This is what makes Lant so broken with its versatility.
One thing to keep in mind when viewing these suspects is how each affects different playstyles. Most balance/stall teams couldn't care less about prankster chansey, but a lot of offensive teams are forced to run a fire type or a guts user JUST to beat it (and make sure the guts user doesn't get destroyed by metal burst). Even voltturn teams have an issue with chansey, as chansey cana often get off chip damage via metal burst in addition to whatever hazards do. On the flipside, a lot of dedicated stall can handle lando with ease, and most offensive teams don't care about a switchin to lando- they don't have a switchin to much of anything, and they most certainly have something to revenge it, while lando t, especially swords dance (or banded), can tear apart balance cores with powered up brave birds or well placed u-turns. Remember to think not just "can this be handled easily" but also "how feasible is it to handle it in different styles/how much does it actually affect different styles." The goal is to make the meta healthy, not just ban things that are strong.
Chansey has basically no counters on offense besides guts fighting types or blaziken, it literally is just a prediction battle on anything that can't OHKO it. The problem with Chansey atm is that it literally switches into almost every mon in inheritance due to the absurd bulk. If its taking less than 66%, it can wisp and the recover off damage the next turn. As a result, it counters half of the meta and checks every non-powerful fighting type offensive mon. If this isn't unhealthy and metagame defining I really don't know what is.
Several things strike me about this statement you made. First off, most offense DOES need checks to common mons such as lant, because they are so centralizing and powerful they threaten to tear apart the team at any moment. How feasible is it to handle it in different styles? This is the issue with Lant: Everything but stall has problems, and a smart teambuilder can even give stall problems. Opportunity cost is everything in teambuilding, but when a mon is this good and has zero opportunity cost since something else can easily fulfill the same role, it is truly broken.
Landorus-Therian
Frankly, I can see Lando-T being more destructive in the future. Sure, it has counters in Regirock, Zapdos, FF Skarmory, and so forth; that being said, I believe it's a lot more versatile than many realise. For instance, its GaleWings set can feasibly be equipped with Will-o-Wisp to cripple common switch-ins such as Regirock and Zapdos, effectively paving way for his team. Lando-T truly has a lot of versatility that has not been explored yet, so that kind of worries me. His Protean lure sets are frightening, and I believe they should be addressed as a problem. You see, they're not exactly lure sets, because they don't really take anything away from Lando-T's overall viability. Those Protean sets are very powerful in their own right. Lando-T is one of the most defining threats in the current metagame, and I believe its centralisation and versatility may warrant a ban in the future; nevertheless, as of now, I don't see how Lando-T is that unhealthy of a threat in the current metagame.
The versatility lant has is exactly the reason it should be banned, and as I have pointed out, the lure sets SHOULD be used over standard because of the ability they possess to demolish teams in an instant. It requires nothing but proper prediction, which is a problem. All of the counters it has are easily beaten by another set.
That being said, you mention how it is frightening how versatile it is, but because those sets are not being used, we should wait. I take exception to this logic because why should we wait for it to be just as broken? If when people start using lure sets it will become broken, why won't we just go ahead and ban it now? The fact that they aren't being used doesn't make the sets less broken. They still exist, and as soon as one person uses it it becomes broken for that person.
This being said, I completely agree with what you are saying about the lure sets, its something people don't really understand because its completely unique.
The difference is that when you run both it's far more obvious, and you give up actual gale wings lando. Since this is inheritance, you can always bop things with a lure set, but overloading the opposing walls has been a teambuilding strategy as long as there's been synergistic threats. Mblaze is a good example of a far more dangerous mon, as it can run the coverage like surf without giving up its main movepool. By running protean instead of gale wings on your lando/archeops you open them up to bring rk'd or checked just by something faster than it/with priority. Having lure sets is something you can't get rid of in a metagame like this, and I'd really like to avoid arguments like that that can lead to banning basically anything just because it has a set and a set that beats that set's checks. Ban targets should be things like keldeo that were hard to wall regardless of set.
This is the exact logic that is why lant is so broken. The opportunity cost is zero. Imagine having 2 lants, one a lure, the other a standard set. Not being able to differentiate between the two, you literally go from hard to counter to impossible to counter. The problem is
this is what exists right now. You can literally run copies of the same mons between archeops and lant or even staraptor, and you can't tell the difference between them until you face them. Its almost as bad as not having a species clause for offensive teams when facing it, and is obviously a problem for stall as well.
Archeops
Archeops is very similar to Lando-T. Most of what I said about Lando-T is true for Archeops. Archeops' much higher Speed stat allows it to be more effective than Landorus-T with non-GaleWings sets in my opinion. Archeops' durability is abysmal, which is essentially the only aspect that is keeping it from being outright overpowered. Despite what I just said, I believe Archeops is more or less as centralising as Landorus-T.
I completely agree with the notion that Archeops has the same problems as Lant, and a slight loss of bulk doesn't change that because Lant was never attempting to bulk hits all that much anyways. TBH Gale wings Archeops is probably better since it takes significantly less damage from extremespeed, and it can actually RK Mega Blaze without killing itself.
Inheriting from Sableye
My thoughts mirror Yoman's thoughts earlier. There isn't a single Pokemon that can use Chansey's most dangerous move, Metal Burst, as well as Chansey herself. Let's give Sableye some time.
While Blissey isn't as good as Chansey is, it is easy to forsee mons that are almost as efficient to the point of just as much centralization. Prankster wisp + recovery and metal burst is such a lethal combination its hard to beat on any wall with good special defense and high HP. Im willing to wait until after a Chansey ban, however, because its not nearly as bad (offense isn't resorting to guts fighting types that can OHKO chansey).
Assist
I am tempted to say ban it because its partially uncompetitive, and I am tempted to leave it in for a suprise factor and a usable gimmick. I really don't know yet on this one. What I do know is that it can absolutely demolish many teams it should not be able to, and without an abundance of flash fire it is hard for balance to wall. I don't think it is worthy of a quickban, but I believe it needs further inspection prior to a final verdict being conceived.
And yeah, offense is hard to run atm. Offense certainly isn't unviable, as adrian put it, but it is very difficult to properly use.