I hope there's an engaging post-game. Without knowing the story of the game, I don't really know what I'd like to see after it. I guess after I know more about that, I can better speculate what I think they'll do. But what I can tell you now is that I do NOT want a giant level spike like in BW. Difficulty and challenge is fun. I would know! But not when it's unfair and unexpected like the post-game of BW. If you're gonna hike levels up, do it gradually so that we can level up as well.
I also would like gym rematches in the post-game, as well as a Battle Frontier of some sort. As for any sort of post-game story, idk. Just as long as it's not lackluster like BW's! Give us something to do, please! Even legendary hunting, like in B2W2. It gave us lots of non-Unova legendaries to go after. I think Deoxys and Mewtwo would be great, if the genetics theme turns to be true. I know Deoxys has always been event only, but wouldn't it be great to have a throw back event to go to Birth Island and get Deoxys? I think it'd be fun. Unlikely, since Birth Island is in the Sevii Islands and we don't know where XY region is, but still. It'd be really cool.