Team Galactic will be the ones studying the Dynamax phenomenon to find a way to exploit it but you will not be able to use it until pretty far in the game. (I do not know if Professor Rowan will teach the protagonist some details or if Cyrus will be the one explaining everything through the game in each encounter with team galactic).
Puffin making and the pokeball capsules are returning. I do not know if dressing your pokemon for the contests will be a thing.
The gym leaders stay the same though there will be two different orders you can fight them after beating your third gym leader in Veilstone city.
Gigantamax Garchomp looks like an F-35 sort of jet
Gigantamax Torterra looks like an Euoplocephalus with a huge mountain on its back
Gigantamax Empoleon looks like a pirate (he looks like he has a pirate costume)
I do not know how gigantamax Infernape will look like.
My source told me nothing about mega evolution, my guess it will not be returning but that is just my opinion.
I’m sorry it isn’t much but that’s all I got.