OU - No Preview

[Snaquaza]

While people already mentioned Wobbuffet, I think Gothitelle might be the more threatening Shadow Tagger. I believe this because it can take out whatever essential Pokemon in normal battling already, but here it can do it without the opponent being able to know the possibility of it switching in. As soon as it gets in safely, it can start wrecking the opposing party with STAB moves, RestTalk, Toxic Stall, Trick or specific moves for specific 'mons.

That said, you can also just ban Shadow Tag. Generation 3 and 4 did the same (This is why Wynaut was Ubers). This will prevent both Gothitelle and Wobbuffet from being the huge uncompetitive threats that they can be in this metagame, allowing more creativity and more room to build.

While both of these are huge threats, I'm gonna look a bit deeper in what they can do.

Wobbuffet: This monster can destroy usually one Pokemon per team, preferably offensive. This is because of the annoying combination of Encore, CounterCoat and Shadow Tag. This allows it to come into anything, trap it and if its choiced, eliminate it if it can't OHKO. If it is not choiced, you can encore the incoming move. If it doesn't 2HKO you can usually KO them because your high HP but low defenses cause you taking a lot of HP damage, but relatively few of your total percentage. If the opponent uses a status move, you can switch out and gain momentum as you get a free turn. However, it can usually only take out one Pokemon before being KO's and most offensive teams have multiple win conditions that prevent them from being overwhelmed.

Gothitelle: On the other hand, this can cause major troubles for defensive teams as it can trap a Pokemon. Afterwards it can Trick a scarf on it or hit it with a supereffective STAB (MegaVenu/Amoonguss). This is not all it can do though as with Toxic it can stall out and eliminate important stall core members, most notable: Quagsire. These 'mons don't have enough offensive presence to break through a set with Toxic and RestTalk. This set, however, is nigh useless against offense. Due to its larger movepool it can easily adapt as well. Trouble with Skarmory: Thunderbolt. Can't break through Heatran: Hidden Power Ground.
It's not that useless against offense either, with a Scarf set it can quickly remove a Pokemon like Wobbuffet, though it's more specific in what it can beat. The Specs set can punch holes, but is eventually mediocre due to better wallbreakers.

TL;DR Both of these threats are extremely hard to handle for a certain playstyle when you can't predict them coming. While they aren't as good against others, they are excellent at what they do with Goth being a bit more useful than Wobb.

 

[vyomov]

Apart from Shadow Tag, one of the key elements of the meta imo is surprise, especially since, say for instance, a Band Cube set kept until the end can wreck a team unprepared, which is why I think strategies such as DragMag will actually do well in this meta.
One mon that I think really benefits from the lack of Team Preview is Aegislash, for two reasons. Firstly, it's versatility means even if it switches in, it's difficult to figure out what sort of coverage it is running(or whether it's SubToxic), and since Aegis can be fit in on most types of teams effortlessly, it's difficult to ascertain just what playstyle your opponent is using. Secondly, Aegis has King's Shield, which allows it to scout out surprise sets more readily, which is an added bonus.

Another element that will dominate is phazing, simply because it allows you to catch a glimpse of the mons your opponent is running, ensuring that you're less open to a surprise set/trump up your opponent's sleeve. Red Card will probably become a crucial element in the current meta, also because it allows you to escape Gothitelle and other Shadow Tag abusers.

 

[xJownage]

stall and to a lesser extent hyper offense just got shit on. This metagame will promote balanced teams with trappers, wallbreakers, sweepers, tanks, and counters to good pokes. hyper offense just got much less neutered especially with gothielle (Scarf set SUCKS imo) and the very potent zoroark. Stall just got destroyed. the prevelance of unpredictable trapping can allow offense to destroy key parts of defensive cores easily, such as mvenu being destroyed by goth to clear for mega mawile, or magnezone being used to beat skarm to pave the way for mega pinsir. Or wobb being used to rk scarfers such as garchomp, paving the way for mega charizard Y. Common stall builds cant function without a key part of their core, and the necessary component can be taken out easily with the stall player unable to see it coming, making stall pretty much unviable since it cant reliably keep its core intact.