Character customization HAS to return. I'm not sure if it was part of the French boutique theme or not but I sincerely hope it isn't just restricted to
Secret Bases, honestly I'll be astounded if they do not come back. I personally never really bothered with them that much but the support for them has been massive since R/S. I'm gonna say I'm 90% certain they're going to return and they will probably be way more extensive than R/S. I'm expecting Global Link synchronization, a huge catalog of items and maybe even the ability to put your base just about anywhere. I'm guessing there are going to be loads of DLC items or just plain old giveaways.
New Mauville is a must. I'm hoping it'll be much bigger this time and will play a much more important role in the game. It was one of my favorite locations in the originals.
Pokenav/Trainer's Eyes and by extension trainer rematches NEED to come back. I'm guessing it will and that we'll get skins for that too. Pokenav may end up serving a similar purpose to the Holocaster.
Pokeblocks and Contests will probably come back hard. I also think contests will integrate Trainer PR videos somehow.
It'd be nice if Mirage Island made a return. I was never able to get there in R/S.
I'm expecting the return of the e-reader and either something to scan QR codes or something that uses TCG cards as AR markers.
The battle frontier will no doubt be the all-singing all-dancing battle area this time around. Given how keen on the metagame GF have been since R/S I'm expecting this to be REALLY expansive this time around. There will probably be something for all skill levels here as GameFreak have been pushing the idea that kids should learn how to battle tactically since BW2 with Pokewood Studios. I wouldn't be surprised if the battle frontier was unlocked early on and had some kind of Join Avenue type mechanic where you could basically build different parts of the area as you play through the game.