I have some ideas...
Liberty Island: (This idea is featuring the idea of not being able to get Victini until Post-game) During the game all you do is go into a tower and capture Victini. You could really do much more with this. I wish that after you met Victini in Liberty Tower you had to chase it all around Unova to catch it, that would make it more interesting.
Nimbasa City: Well, we have the Ferris Wheel, but you don't really get to hop in and go around in it, that would be awesome. Yes, you can get in it, but a post-game player-in-the-wheel perspective of Nimbasa would be epic.
Desert Route 4: Okay, so we have all this building, maybe in Post-Game the construction is completed and you can buy a house there or something....
Desert Resort: Maybe after you get one Darmanitan out of its Zen mode you can find more RageCandyBars and be able to get the others out of Zen mode, because leaving those others there is an eye sore....
Musicals: Post-game it would be cool if you could put your pokémon in musicals that resembled what happened and honoring your pokémon as a hero at the end, with a large spotlight and lots of clapping.
Undella Town: I think in the Winter (post-game) the waters should freeze over that way Abyssal Ruins can only be reached during the Spring, Summer, and Fall, thus making it harder to get Arceus' plates. Also, you should be able to explore and figure out what the torrent is!
I could go on and on, but here's this....