Pokemon Scarlet and Violet are quite ambitious titles in that they are the first Pokemon games to be fully open-world, meaning that there won't be loading times while exploring the world - except perhaps when flying or fast traveling to other cities, but that remains to be seen. Pokemon Scarlet and Violet could also be a soft reset for the series, in the sense that Gen 9 games could be more similar to Gen 1 and Gen 5 in terms of their regional Pokedex size and approach to the national Pokedex. It is also likely that Generation 9 will feature regional forms again, as they became a staple ever since they first appeared in Pokemon Sun and Moon, even returning to Pokemon Legends: Arceus.
A known leaker that goes by the name of Riddler Khu shared information regarding a new set of forms rumored for Pokemon Scarlet and Violet that could go by the name "regional fakes." This stems from a riddle that required fans to decrypt a string of code several times before getting a short sentence that said "F means fake," which ties back to another teaser suggesting the alleged regional Diglett in Pokemon Scarlet and Violet may have some truth behind it. That regional form was not real, but its design happened to be similar to the idea of a regional fake, according to Khu. However, the question about what regional fakes are remains.
The Possible Meaning of Regional Fakes in Pokemon Scarlet and Violet
The Diglett design that was shared online as a supposed leak, although it turned out to be not real, depicted the Pokemon as so massive that the critter had its head in the clouds. Considering Khu described this leak as close to one of the regional fakes in Gen 9, this detail could be pointing at the return of Dynamax and Gigantamax Pokemon - just not as a gimmick players can actively use. Dynamax and Gigantamax Pokemon are massive as well, and have small red-and-purple clouds floating around their heads, resembling the fan-made Diglett.
What this means is that Pokemon Scarlet and Violet could feature critters similar to Dynamax and Gigantamax Pokemon in terms of appearance, but upon defeating them or catching them they would lose this ability - thus, regional fakes. The phenomena of Dynamaxing is exclusive to the Galar region, as far as Pokemon Sword and Shield's lore suggests, so it wouldn't make sense for these critters to appear elsewhere.
However, Pokemon Scarlet and Violet could make past gimmicks come back in some way by having wild or trained Pokemon that do display peculiar characteristics based on those gimmicks, even if they can't really use them at their full potential. This could apply to Mega Evolutions, or maybe even Z Moves, but it's not likely to apply to battle gimmicks like double or triple battles, or anything else where the Pokemon involved don't change their form in some way.
Ultimately, this is just speculation, but it could also be that "regional fakes" are simply Pokemon who can change their form while battling but not their base form when in the overworld or outside of battle - not unlike how Zoroark's Illusion ability works. In such a case, gimmicks wouldn't be involved at all, and Pokemon Scarlet and Violet's own battle gimmick could be allowing Pokemon to change their appearance under specific circumstances. More details are likely to be shared in the coming months, so fans will have to just wait and see.
Pokemon Scarlet and Violet release in late 2022 for Nintendo Switch.