To me, these would be awesome.
1. Being able to go to other regions post-game. Seriously. That would definitely make the game last longer and more fun due to the fact that we could become Champion there, and just have an all new story-line for old regions.
2. Gym leader rematches. That would be wonderful, especially if they'd also have a mega evolution. And that takes us into...
3. More mega battles. I know mega evolution is supposed to be special, but no NPCs, other than story-line characters, E4, and champion, have it.
4. Harder NPCs. The NPCs have become way to easy. They have only one or two pokemon (three at most if on VR) and usually they can be K.O.'d with one hit.
5. More customization. That was my favorite thing about XY, the not-already-chosen character. If we could still have that, along with maybe more outfits, hair styles, etc, along with stuff like that for pokemon, that would be awesome.
6. Pokemon walking with you. The whole reason I bought SS was because of the fact that your pokemon walks with you. It really shows that 'bond' everyone in game keeps talking about.
7. NPC rematches. When making a new team post-game, it becomes really difficult to receive EXP points because you've defeated most NPCs. So either add rematches, or at least give more EXP from wild battles. (And money as well. How can I catch 600 pokemon with 10 poke dollars?)
8. New story-line. It's all the same; evil team using a legendary to achieve their goal. It's boring and predictable. New stuff please! Like, what if Maxie or Archie won? Or if Team Plasma liberated everyone's pokemon?
9. More post-game stuff. Once you've beat the game, all you can do is complete the pokedex, and when that's done, you have nothing else to do unless you reset the game.
10. More rival stuff please. I personally love the rivals, and feel like they get to small of a role. Especially post-game. It's like they disappear. At least have activities like battling, trading, or talking with them. And put them in the story line too.
11. (last one) Better in-game trades. Some people can't trade via Wi-fi or Spotpass. So, how can they get unobtainable pokemon for their pokedex? And not just that, many people refuse to trade good pokemon. Most GTS and normal traded pokemon are low-leveled. Not only that, but in the GTS, people are asking for Mews and Genesects and Hoopas for everything. We don't have them either. Get over it.