Time to refute some more DNB arguments.
All you need is one KO to already have the advantage. If you can eliminate the check to a specific Pokemon, the opposing team will have a much more difficult time trying to take it out, and a lot of times the team will start to hemorrhage under the pressure due to lacking a check of the Pokemon you're disguising as.
Illusion can re-activate later in battle and due to bugs Illusion can make you appear as different Pokemon mid-match. The only way to beat Illusion is to make the Illusion Pokemon faint. Prankster SubPass is definitely niche, and while it works well, you can only get so many uses out of it. Of course, the opponent is simply going to stop switching in their Illusion Pokemon into your Prankster mon once he learns about it. Why switch in Illusion when Illusion can't beat that Pokemon?
Also, scouting for Illusion isn't really possible unless you see the opponent has a normal Kangaskhan, and next to no one at high ladder leads with an Illusion Pokemon because of Imposter Chansey with Fake Out. Using Prankster Sub does not count as scouting for Illusion because some times it may not be terribly obvious that the opposing Pokemon is Illusion (e.g. when Mega Rayquaza uses Moongeist Beam, is it Mega Rayquaza or an Illusion Mega Gengar?).
Again, we're pointing fingers at a different issue to try and justify Illusion's place in the meta. Other things being an issue should not have an effect on this Suspect Test. Imposter Chansey is not broken either in my opinion. The sole reason is because it is entirely one-dimensional and if you take the time to improof your team, Imposter may have little to no impact besides healing up, but even then you can get rid of it easily with trapping moves, namely Anchor Shot. Regardless, if you believe it is a problem, I still believe you should post about it in the Bans thread, not here.
Also, in my honest opinion, decent teambuilding will let you beat Imposter, but I completely disagree that decent teambuilding allows you to beat Illusion. There are OMPL games where Illusion has worked out well. GL Volkner won an OMPL game using it. Motherlove and Willdbeast have lots of games using it in OMPL.
Want the replays? Have them.
https://replay.pokemonshowdown.com/gen7balancedhackmons-773107894
https://replay.pokemonshowdown.com/gen7balancedhackmons-779377481
https://replay.pokemonshowdown.com/gen7balancedhackmons-766680809
https://replay.pokemonshowdown.com/gen7balancedhackmons-766658509
https://replay.pokemonshowdown.com/gen7balancedhackmons-688449709
Don't lead with Illusion and don't send them out on things that utilize FakeSpeed. Problem solved. Illusions strength is targeting specific checks to remove them from the game. There are few things Illusion can't do but sending it out against Pokemon who utilize priority is one of them.
Of course, you won't be able to utilize things like Illusion Registeel effectively. Illusion would go good on any sort of wallbreaker. The only way you can really revenge kill Illusion Pokemon is by Pursuit trapping them which works on Mega Gengar but not on many of the others. This assumes you have a banded Tyranitar or Gyarados on your team to begin with.
Also, as Illusion re-activates later in battle after you switch in again and the Illusion can be disguised as a different Pokemon (first time can be Mega Diancie, the next can be Giratina), I'd argue it is not a one time advantage. It can definitely be abused multiple times during the midst of a game.
The meta is already particularly unpredictable even without Illusion factored in. Unless the Pokemon is resistant or weak to Stealth Rock, you won't be able to tell if it's Illusion most of the time, and if the opponent was dedicated enough, they could keep hazards off the battlefield especially if they were also using Shedinja. As for Spikes, every Pokemon but Flying Pokemon takes the same amount of damage.
I genuinely ask that some of you reconsider your Do Not Ban stance here. Illusion may be harder to use on ladder than many might think, but when you find the right times to send out Illusion and the best Pokemon to disguise as, it can be deadly in OMPL matches. This isn't to say that ladder doesn't have trouble with it either. I have used it many times and so many times I have seen the opponent make a bad move and end up giving me one or more free turns because of Illusion, like Volt Switch on an Illusion Mega Garchomp. It is completely unfair and uncompetitive at the basic level. It may not be overpowered to many people, but if you find out how to use it correctly and efficiently, it ends up giving you more of an advantage than any other ability could due to it giving you free turns to attack or set up without the threat of Imposter Chansey.