I like the O-powers. I never used them playing through the game, but they're great for extra experience points and egg hatching when you're building competitive teams.
I definitely agree with the Contests...they NEED to come back, and I'd like to see them come back REALLY well. It will give people who don't like competitive battling another avenue to pursue, allow pokemon to be picked based on favourites and not their battling and make never used moves suddenly useful. Gen 3 had the best contest system, and I'd like to see it crossed with the pokemon Amie in someway. I love pokemon Amie! I want it to stay! Though, I don't need the decorations as much as just the interaction. I'm sure secret bases will be better for decorating.
And BATTLE FRONTIER needs to come back. Enough said.
And removable hats would also be fantastic!!! I was also really surprised that they didn't have more hair colours and options, especially for dudes. There was no curly hair!!!
I can really understand if they don't have your pokemon follow you, just because the amount of work it would be and just how much it would require of the cartridge. I can't help but feel, however, that some of the mechanics in X/Y are teasers for things that are gonna be expanded in X and Y...
Like sky battles were generally lots of fun, and would make a great addition to the Battle Frontier. I also think it could be a strong hint at Dive battles, as someone already mentioned, using only pokemon that swim underwater. It could make all that water we have to swim through more interesting. I'd personally love a HUGE battle frontier, with some of Sinnoh's frontier making a comeback. I wonder if inverse battles in X/Y would be in the OR/SE remake in the frontier, and that one battle was put in as a hint???
I also wonder if instead of pokemon following us, they'll let us ride certain pokemon that we actually own to make up for this, and maybe to replace the need for two different types of bikes. I always found that REALLY annoying in the original RSE games. They've already given us a taste in X/Y and they've had time to improve and pick more pokemon for artwork. We also got to ride our own lapras in this way, so I think it's entirely possible. I think we might even see MORE water pokemon with original sprites for Hoenn because, well, SO MUCH WATER.
I just can't help but feel like riding pokemon in general might be a bigger thing... like maybe pokemon races. After all , in X/Y your mom is a ryhorn racer, their have been several race episodes in the anime. Almost like the pokeathalon in soul silver was added, maybe we'll see races in Hoenn. I mean, they basically invented Gogoat to be ridden. I think it would be a great feature and would kind of make it up to all the people who want to see pokemon follow us again, even though I'm pretty sure it would be WAY to complicated.