I agree with the people who say that Illusion was the wrong thing to suspect this time around, but as they are also saying, we should discuss what to suspect next. And though I did write a long post about Sleep clause, and I believe Sleep Clause is probably a good step as well, there's something a lot more serious we should deal with first:
V-Create
Yes, V-Create. Not Simple or Contrary, or Shell Smash or DQM.
Why V-Create?
It's by far the strongest move in the metagame, with only the unfortunate side effect of lowering the speed and defenses of the user.
It's as strong as or stronger than many crystal-powered Z-moves, but doesn't require a crystal of its own, and has 8 pp. We banned CFZ's before, and they only had 1 PP, so it's like being able to use a CFZ 8 times at the cost of some stats which you can just switch out of to reset.
It hits all relevant Steel-types except Dialga super-effectively for ridiculous amounts of damage. There are other moves to hit steel types, such as precipice Blades, Close Combat or mind Blown, but even the strongest among these has 16% less power than V-Create (we will ignore that it's a special move for now).
With contrary, the negative of V-Create, lowering speed and defenses, suddenly becomes a positive, by increasing the speed of the Contrary mon to such levels that it becomes more and more difficult to revenge kill with Scarf users. V-Create boosting defenses also can allow it to survive priority moves that it otherwise wouldn't have. For example, a full health mega Rayquaza that has used V-Create one time can live the combination of Fake Out and Extreme Speed from Mega Diancie and win 1v1 if it happens to be running Superpower or Leaf Storm. No other move in the Contrary toolkit can let you outspeed scarfers AND protect you from priority at the same time.
There are still powerful Fire-type moves as alternatives such as Sacred Fire, Flair Blitz, Blue Flare, and Mind Blown, all of which have different ways to deal with them, and pros and cons to using them.
The biggest argument in favor of banning V-Create, imho, is the one about banning how it is analogous to banning CFZ moves. With comparable power, 8 times as many chances to use it, and the ability to be actually BOOSTED by an item, it's just crazy.
Arguments against a ban
It can be resisted. bulky waters like Slowbro-mega, bulky Dragons like Giratina and Zygarde-C, and Flash Fire can all keep it under control. In a vacuum this might be true, but team support can allow you to easily break through these.
It forces you out after use. Offensive teams are not big fans of the speed drop which lets a lot of mons outspeed the user that otherwise wouldn't, and force it out. That's a huge momentum drain on offensive teams especially. The speed drdop dissuades people from setting up and then sweeping through teams with it as well.
Stab users of the move are weak to Stealth Rock. Very true. Most of them can recover their hp in some way. Primal Groudon and mega Blazicon, arguably the two strongest V-Create users, only take neutral damage from rocks though, and Mega Diancie which often runs it as coverage, takes neutral damgae as well.
With V-create gone, we can bring back Primal Groudon. We can revenge kill enemies that use Contrary again either by priority or with scarfers. We don't have to worry about switching our Regenvest Solgaleo into Mega Diancie, and dying to V-Crate which does about 85%).