Also, I cna't wait for Contests. I don't want to see Super Contests, but the usual, RSE Contests. They were a pretty fun sidequest, and, other than the Pokéathlon, have been the only decent sidequest for me in the series. I'd love to see them integrate some sort of animation for the attacks so we can see them in a Contest form.
•Music: I loved the RSE tracks and I'd like to see them remixed.
Please, please, please say the Battle Frontier returns. I loved it in Platinum and never got to properly play it in Emerald and it is one of the few features that I've truly missed in later games. 3D, new Mega Evolutions and customisation should all return and I expect that we'll see more post game stuff as well (With FRLG and HGSS both doing that). Apart from the BF, RSE had a very lacking post-game, so I'd be surprised it more wasn't introduced. Kyogre and Groudon naturally look like getting Megas, but I would really like Walrein, Salamence and Milotic/Metagross get them so that all the Hoenn E4 can have Megas!
One point to speculate - Who will be the champion? Steven is more popular and the obvious choice, but if Wallace is the Champ, Juan can be a gym leader, Steven can hide out in Meteor Falls and all three can be in the game. Shoehorning Juan in might be more difficult if Steven is the champion.
Bring on the Trumpets!
I think Secret Bases transferred to others via Street/Spotpass would be cool. Maybe if you find it, the other person can leave something inside, maybe a Pokemon or an Item... I dunno?
I hope that the trick house will be revamped and also remain active even after facing the last riddle, because honestly, I really didn't like that the trick master disappeared just like that...