I usually don't like talking about this type of stuff, as I have loved everything. And most people usually say, Ash not being dumb anymore, Misty or Brock or something along those lines
But things that I miss the most... I really don't miss anything. A lot of stuff I used to love, they brought back in XY Saga. Pokemon having their own personalities. Feelings that involve crying, ect.
I guess something I do miss and Pikachu's love for ketchup. Apparently there has only been 1 or 2 episodes about that, but I remember it the most for one reason or another. Other things I miss are when Pikachu is doing things in the background that are generally funny when the trainers are talking. He just doesn't sit on Ash shoulder. Other things I miss are the comedic things. One last things I miss is references to the real world. Like how they had a few episodes based around Christmas. Although they somewhat brought that back in XY as well. I really miss those type of episodes.
One of my favorite episodes in the XY series was Gourgeist Festival. I really love Halloween. Even thought the Gourgeist Festival wasn't 100% Halloween, you could obviously tell that it was inspired and based on Halloween. I really want to see a Christmas inspired episode. It doesn't have to be 100% that, but even if there was just a Winter Festival, I would really enjoy it. I was hoping we would get one around Snowbelle City. Alola would have to be late in the saga probably. The only snowy areas is on the mountain.