Triggered
Basically all abilities that trigger due to certain circumstances will cause the Pokemon to use it's first move in its moveset instead of the ability's effects.
All moves that are always active like Huge Power will just do their normal thing (in this case doubling the Attack stat)
Example: Let's say my Lucario with Justified gets hit with a dark-type move. This normally would trigger the ability and raise Lucario's attack. However in this metagame, it will instead cause Lucario to immediately use the first move in its moveset. Let's say Lucario's first move was Nasty Plot. This would effectively turn Justified into a special-attack raising version but with double the effect. This is just an example, I don't think this would be very effective.
This can make weather or terrain-starting mons like Pelipper or Rillaboom quite effective. Send out Rillaboom, it immediately uses Swords Dance, effectively giving it Huge Power. Coupled with its high Attack Stat, this could make a very strong threat.
Banned Abilities: Abilities that trigger every turn (Speed Boost, ), Stance Change (Therefore Aegislash is banned), Ice Face (Therefore Eiscue), Disguise (Therefore Mimikyu), Libero (There aren't any Protean mons in Sword/Shield)
Banned Moves as 1st slot: WIP
Banned Moves in general: All Switching moves except Teleport,
Potential Threats:
Weather/Terrain-starting moves, other moves that activate upon entry like Intimidate, Magic Bounce, Stat-boosting moves.
Potential questions:
Does it consume PP when the Pokemon uses its move through the ability?
Yes.
I think that X should be banned.
That's not a question. Also feel free to say what moves should be banned.
What about Choice Items and Assault Vest?
If a move is used through an ability and the Pokemon hasn't used a move yet, Choice Items will cause it to lock into the move it just used. If it activates again after it moved already, then it either uses the move it was locked into or the first move in the moveset. I don't care, either would be interesting. The same goes for Gorilla Tactics.
For Assault Vest, it works as normal, but the move fails if the first move is a Status move.
EDIT: Can you still use the move in the 1st slot?
Yes! You can use it as much as you want (assuming it still has PP, that is).
EDIT: Does the ability still cause the normal effect?
No. If you were paying attention then you would've noticed that it takes effect instead of the ability's normal effect.
This is a cool Idea in concept, but in practice, I think it falls apart for 2 main reasons.
Firstly, is the problem of abilities that proc when switching in, which is the entire gimmick of Trademarked. Allowing this is just gonna make the meta "Trademarked Lite", but with a much more limiting list of mons at one's disposal.
Secondly, if you ban all abilities that proc on switching in, then the meta's gimmick becomes reactive rather than proactive,
the opponent needs to use a sound move so your Kommo-o can Soulblaze on it,
they need to use a ground-type move so Rotom can defog on the switch in,
they need to use a physical move to proc your Corsola-Galar's Strength sap, etc. And if you guess wrong, then you're just playing OU, and that defeats the entire point of it being an OM. On top of this the abilities that are reactive are either annoyingly restrictive and specific like with most, if not all, immunity-based abilities, or way to all-encompassing, like with Iron Barbs/Rough Skin, Weak Armor.
I also haven't even touched on the prospect of having actual attacks as moves that can be activated, which seems very cool, but, once again, falls apart. If abilities that proc on the switch are allowed, then the game devolves into +6 priority attacks(the priority for switching)/revenge kill: the game. Even if you disable the ability for a pokemon that have an ability that activates on the switch in to activate when coming in on a fainted pokemon, the opponent is just gonna send in their fastest pokemon to have a higher speed and hit with their +6 priority Cloud Nine Drampa Draco Meteor/Banded Tyranitar Stone Edge/Whatever.
On the other hand, the reactive abilities, such as Weak Armor, Volt Absorb, Magic Bounce, etc. twist the game akin to how Sm4sh Bayonetta twisted her game with witch time. The opportunity cost of using an attack might just be heavily outweighed by the prospect of actually just dying, Rotom-Mow with Leaf Storm, Ferrothorn with Body Press, Specs Hatterene/Espeon with whatever. I know I mentioned the difficulty in using more restrictive abilities like Sap Sipper, Flash Fire, Overcoat, etc. But the principle still applies with these very specific moves, and when there's a team full of them, they aren't as restricting due to the 5 other teammates with other immunities.
The list goes on, and I feel it will lead to a very stally meta where only the safest attacks can ever be made, or an HO nightmare where games last slightly longer than 1v1 because everything is switching in and out using their strongest moves without a second thought.