Special Reunion
In the transition from Pokemon RGBY to GSC, the split between the Special stat into the Special Attack and Special Defense was made to balance Pokemon such as Alakazam who had its Special incredibly high. For Kanto Pokemon, they either had identical SpA/SpD (such as Dragonite), gained in one or the other stat (such as Snorlax), or took a lose in one of those stats (such as Alakazam).
In this metagame, the SpA and SpD of all Pokemon will be unionized by the average (rounded down) of their Specials into 1 Special stat, and moves that boost either will be boosting by the same stages but for just the Special stat.
As an example:

In this meta, Togekiss will have the following stats
85/50/95/
117/80.
Here, instead of having a 120 Special Attack or Special Defense 115, Togekiss now has a 117 Special instead. This is because the average of 120 and 115 is 117.5. This is rounded down to be a whole number.
Let’s also examine some special (pun intended) cases with moves, abilities, and items:
Boosting moves that boost one stat:
Moves such as Nasty Plot, Amnesia, and Charge Beam will boost the entire Special stat by how many stages they raise their respective stat. Nasty Plot will raise Special by 2 stages, Amnesia will raise Special by 2 stages, and Charge Beam will raise Special by 1 stage.
Boosting moves that boost both stats:
Moves such as Calm Mind, Quiver Dance, and Geomancy will simply have the boosts condenced. Calm Mind will raise Special by 1 stage, Quiver Dance raises Special by 1 stage and speed by 1 stage, and Geomancy will raise Special by 2 stages and Speed by 2 stages.
Shell Smash:
Due to Shell Smash decreasing Special Defense while increasing Special Attack by 2 stages, Shell Smash will instead increase Special by 1 stage. The reasoning is that when Shell Smash is used, it will have a turn that would decrease Special by 1 stage (when subtracting the user’s bulk stages), then increase Special by 2 stages (when adding the offensive stages). This just nets the user with +1 Special each use of Shell Smash.
Ice Scales:
Due to its fuctionality, Ice Scales will reduce Special damage, not double your Special.
Eviolite:
The same thing applies. Eviolite reduces damage, not increasing your SpD. You can actually see this in action as Body Press is not boosted by Eviolite.
Edit: Apparently, Eviolite does indeed boost SpD, and it’s just that Body Press has a weird interaction with Eviolite and Choice Band.
Choice Specs/Wise Lens:
This item, like with Choice Band/Muscle Band, increases the damage of moves, not your actual stats. Again, you can see this with Body Press being more powerful with Choice Band/Muscle Band.
Assault Vest:
It’s pretty hard to say for sure, as there is no Def version if Assault Vest. However, with Psyshock/Secret Sword, we can see that Assault Vest does indeed increase Special Defense, so it will increase the Special Stat by x1.5.
Sand Stream/Sandstorm Weather:
Similar to Assault Vest, this increases your Special Stat instead of decreasing special attacks, so it will increase the Special of Rock types in Sand by x1.5 as well.
Light Ball/Clamper Items:
Light Ball will increase Pikachu’s Special by x2, and so will Deep Sea Scale/Tooth for Clamperl (if it does get released in the future)
Potential Bans and Threats
Assault Vest is essentially a Choice Specs that reduces Special Damage and has a much kinder restriction than being Choice Locked. The only setback is that you can’t use Amnesia or Nasty Plot. It will easily be the top of the threat list and likely banned early on.
Amnesia/Nasty Plot/Cosmic Power:
With Amnesia and Nasty Plot, you are essentially giving a Pokemon 2 Calm Minds in 1 turn, while Cosmic Power is essentially Calm Mind if it boosted Defense as well.




in Sand:
With Sand, Rock types get a free Special boost, and these have excellent Specials as well. Diancie being the biggest threat here as it gets Amnesia, an amazing Special movepool, and a strong Def.


:
Both packing Unaware make them incredible at beating opposing Amnesia abusers. You either have to beat them before they boost, bypass their Amnesia boosts, or hit them physically.

:
Even with a Special stat if 66, its Simple Nasty Plot, Timid gives Swoobat 693 Special, which is higher than 255 with Modest, and almost reaches Blissey’s HP. It does have weak physical bulk and Stored Power won’t be as strong as its regular Calm Mind.
Questions:
-What restricts should be added to Nasty Plot, Amnesia, and Cosmic Power?
-How should Assault Vest and Sand be handled differently?
-What about Natures? Especially Careful and Rash?