Theories for Pokémon Scarlet and Violet are likely anything but accurate at this point, but one that has some weight is a new transformation gimmick. There are likely to be some new features in the Gen 9 games when they release later in the year, and one could allow Trainers' Pokémon to take on new forms like Mega Evolution or Gigantamax did. It's far from a certainty at the moment, but the thought is still very exciting.
In Pokémon X & Y, the series introduced Mega Evolution, a method of transforming fully-evolved Pokémon into a more powerful form by using Key Stones and Mega Stones. Sun & Moon brought regional variants, alternate forms of previously-introduced Pokémon that are exclusive to their home regions. Sword & Shield has Dynamax and Gigantamax, which gave even more Pokémon advanced forms as well as ballooning them to titanic sizes. Legends: Arceus brought Alpha Pokémon, which may not be as dramatic as some of the others, but is still a change in its own way.
Related: Pokémon Scarlet & Violet Version Exclusives Are A Terrible Idea
With all of the transformations and alternate forms that the Pokémon series has been introducing ever since X & Y, it's not outside the realm of possibility that the next games, Scarlet and Violet, will have their own. What exactly it would be is uncertain at the moment, but hopefully it would be just as exciting as Mega Evolution. The games' close release date means that news could come at any time, and if there is a new transformation method in the same vein as past gimmicks, it's likely been in the works for a while.
Not every Pokémon game has Mega Evolution. It was missing in Pokémon Brilliant Diamond and Shining Pearl, despite the fanbase clamoring for it. It's come to Pokémon
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