In my opinion, I feel that if the developers are going to continue making gimmicks, they should make a gimmick that could easily be carried over to future games and still be relevant. If not, this could ruin the gameplay of certain Pokémon.
For example, while Tera Types are a good gimmick, if they don't appear in future games, then moves such as Tera Blast become borderline useless, and Pokémon like Ogerpon and Terapagos can't reach their full potential again. Are we just going to completely leave these Pokémon out of future games, put them in future games without their gimmicks to make them powerful, or just wait until the next Scarlet and Violet remake to see them again, which might occur a decade from today?
I believe the next gimmick should involve Pokémon abilities, and it can be called something like "PokéPowers." A Pokémon on the field can have its abilities upgraded for the duration of the battle. For instance, Pokémon with the ability Drought or Drizzle could tap into their "PokéPowers" and make them similar to the abilities of Primordial Kyogre and Groudon's abilities in the Gen III remakes. Pokémon with Intimidate could tap into their PokéPowers, activate it again, and this time, it would sharply lower both Attack and Special Attack with every switch in.
With abilities like Libero and Protean, after the Pokémon activates its PokéPowers, it can return to being how it was before the nerf in Gen IX, where it changes type every time it uses a move of a different type. Abilities such as Long Reach could hit Pokémon that are using moves like Dig or Fly. Abilities such as Poison Point, Effect Spore, Flame Body, and so on work 100% of the time after the opponent makes contact. You get the point.
In addition, I say bring back abilities with field effects too using this power. Since we have Pokémon roaming the open world now, let's make it interesting. Let's activate Intimidate in the open world and scare away all the low-level Pokémon and leave the high-level ones. Let's make Stench not work like how Repel worked in other games, but make it so that aggressive Pokémon stop chasing us. Let's make Cute Charm attract Pokémon of the opposite gender again.
Regarding the discovery of this "PokéPower" gimmick, what better character can do this than Colress, the guy always talking about bringing out the full potential of Pokémon? Supposedly, a Unova remake is coming up, so this is the perfect time for Game Freak to make this happen. This gimmick is not too complicated, likely easy to program, and can be safely used in future games without disrupting gameplay mechanics and invalidating certain Pokémon.