A bunch of them have been already said. No BF would be quite disappointing, I would sink time into that for sure. Stuff like mega flygon would be a disappointment, just less of one.
I would like to see more of Wally, and for the rival (so May or Brendan) to actually be a rival, not someone who rewards me all the time, then gives up in their own quest by Lilycove and tries to give me advice on beating the champion every single time regardless. =/ But maybe I'm hoping too much there.
My main hopes with contests being back in RSE style and Secret Bases being in with wifi support are already fulfilled, so I'm happy with that. I think the Amie will be back in (they were showing off the pikachu cosplay forms in that after all), I'd be happier if the trainer customisation was back in with more options for males (we have a far smaller range compared to female players), and I also don't at all mind Pokemon following you, but I find it unlikely to be put in (it hasn't been shown yet as pointed out, and would be even more work than in HGSS - you'd need models for 700+ Pokemon, extra for forms, and that's just animation.)