I'm hoping that they keep the third tier promotion, though I hope they can make the game not grindy because grinding in a normal fire emblem playthrough just doesn't feel right to me. I also hope they will keep the weapon triangle. As for classes, i'm okay if they're going to make new classes, but I hope they will be more unique than the ones in fates (like blacksmith basically being a hero who prefers axes, and etc.).
Edit: I also hope that characters could keep their special colour pallete whatever class they change into because it has always bugged me how some characters change to the generic blue armor (ex: Effie becoming a blue armored great knight, etc.)