Welp. There have been four leaks already and people are speculating, so I might as well share what I know. Or maybe I just want attention, believe whatever you want.

Wild encounters are like trainer encounters; if a Pokemon sees you, they'll either chase you, ignore you, or run away

Large Pokemon will sometimes charge you from off-screen; smaller Pokemon can be found by examining trees and other random objects

Non-native Pokemon can be found as extremely rare encounters right from the start of the game

SOS calling returns from Sun/Moon, but is only rarely used by unevolved Pokemon to call for their evolved forms not normally found in that area

The map is generally the same, but the areas have been significantly redesigned; you can enter most of the buildings in Saffron, which features things like a movie theatre and a ballet school

Red and Blue travel together and act as your rival, with which one you're actually battling against depending on which starter you choose

The game apparently has a completely original story featuring Team Rocket as the antagonists