Let me name a few that I would love to see.
More variety in the villainous teams. I'm sure I speak for at least a few people when I say that when I battled Team Magma and Aqua, I got sick and tired of seeing nothing but Zubat, Poochyena, Carvanha/Numel and their evolutions. Even their leaders only had the same three Pokémon's final forms instead of something creative like Cyrus, Ghetsis and Lysandre. So here I do hope they have better teams instead of just being a marathon of the same three Pokémon over and over.
Better means of training. Ugh, this was killer with the Hoenn games, the fact that training was an absolute chore. Emerald had absolutely nowhere to level-up on your own, considering all there really was is the Artisan Cave, and lower level stuff in the Sky Pillar. Also, outside of that, it was nearly as bad as Johto with training at points since the wild Pokémon were so terribly underleveled. So i'd imagine they're gonna keep XY's EXP system where everything that enters battle gains equal amounts of EXP, which I would be fine with. And maybe bring back the Lucky Egg? They've had it in every game since BW, so if they keep it here i'd be alright with that. Also, i'm curious as to what they'll do with the Exp. Share.
More post-game. In my eyes, Ruby and Sapphire has THE second worst post-game in the series, only ahead of Red and Blue. Nothing to do other than Rayquaza, Latios/Latias and a Battle Tower that you HAD to be Level 50 or 100 to enter. They might remake Emerald's Battle Frontier, which would be great. Also, maybe something like the Black Tower/White Treehollow that will be fun to explore and convenient for training? I'd love to see something like that.
Oh, one more thing: DON'T BRING BACK SCOTT. Just so idiots don't keep making fun of him. (My real name is Scott so I don't take that joke well, please.) I'm sure at least 90% of those who played Emerald basically hate the guy, ESPECIALLY those who watched Marriland's Emerald Nuzlocke. I didn't really care for him either, he was fat and showed up at the most random times. So yeah..
And those are just a few I can think of. I'm sure there are more, but this is just a start.