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.
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.
How Likely Are Past Transformation Methods For Pokémon Scarlet & Violet?
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 GO recently but was absent in Legends: Arceus, which is somewhat understandable due to its ancient time period. It's tough to say whether or not it will be in Scarlet and Violet, since there's so much that's still unknown about them at this point. If it is, then they could continue the trend of starters gaining new forms, with Gens 2 and 4 being the most logical candidates.
Gigantamax, on the other hand, is most likely not in Scarlet and Violet. The Power Spots necessary to perform it are exclusive to Galar and the surrounding areas, so unless the series decides to spread Eternatus' influence across other regions, it probably won't include Dynamax or Gigantamax. There are other ways that Pokémon can be a little different, however. The Pokémon games' regional variants are a great mechanic and have been in every game since their introduction in Gen 7, so more could certainly come in the next games. Alpha Pokémon are a relatively new development, and a minor one, but they could also cross over from Legends: Arceus.
If Scarlet and Violet have their own way of transforming Pokémon, then it would need to stand out from both Mega Evolution and Gigantamax. Gigantamax set itself apart with its enormous size change, so something that's not quite the same but along those lines could help make Gen 9 unique. Armored Pokémon with special gear to aid them in battle might be a bit of a stretch, but the idea is a fun one. Some Pokémon certainly deserve regional variants in Gen 9, but hopefully, that's not all there is in terms of alternate forms. Anything could happen at this point, so until more information is released, those eagerly awaiting Pokémon Scarlet & Violet will just have to wait and see.