My thoughts on Seviper...
The deal with the standard format right now is that special conditions just aren't viable enough, as there are too many outs to recovering from them. Maneuverability upon the active spot is so prized in this meta, all thanks to the notorious Deciduplume strategy of using Lysandre to trap the active spot.
Even when Vileplume rotates, we'll still have plenty of ways to escape the active spot with ease, such as Float Stone, Escape Rope, Free retreaters, Zoroark, Solgaleo-GX, and the new Guzma card as well. Ariados rotating does not help either, and there are plenty of healing and prevention cards still around, such as Comfey, Magearna-EX, Ability lock, Pokemon Center Lady, Bent Spoon, Chaos Tower, and so on.
In such, there are too many 'outs' in special conditions for any deck primarily based on inflicting them to be viable right now. Perhaps the only deck that can utilize them decently in standard would be Espeon-GX/Garbodor - but that's only because the setup of using Psybeam for early game pressure is ridiculously cheap and effortless to perform!
Now, back to Seviper: All I see at the moment is a 1-prize Regirock-EX, only for increasing poison attacks. Players can easily get rid of poison right now, so the ability is just +10 damage and that's it. So far, it could use a big basic attacker that can simply hit for 20 damage with one meager energy and auto-inflict poison. If this happens, we have a neat strategy: add a choice band, 4 benched Seviper, and you inflict 100 damage. Sure, 50 extra damage could appear if it lingers, but unfortunately, like I said: We have too many ways to nullify special conditions.