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I'm assuming that general new-areas-redone-story stuff comes as default.
1. Improve your friends. Having more of a human element was good, but their implementation was just so flat. Make them less comically incompetent and give them more relevance to the plot. More gameplay integration would be good, too, like them offering to trade / fight / train / hang out with you once a day or something, with more / better options as you become better friends.
2. More to play post-game. Looker was fun (and had surprisingly good characters for a Pokemon game - why couldn't the main game have gotten that?), but was too short and lacked exploration. Something at least on par with the Sevii islands is needed; a whole new and rewarding area to explore.
3. More customisation options, especially unique or costume-like ones that you find rather than purchase. It's ideal side-quest/exploration reward material - I can't believe that they had literally one single piece of clothing that isn't just bought.
4. Better battles - hard mode with higher levels, fuller parties, and strategically-built teams you have to counter to beat without massively overleveling. Doesn't need to be anything too over-the-top, just stuff like "use a Substitute" or "get rid of their weather" or "catch one of the two Unaware mon we conveniently put just before this gym". Rematches would be nice, too.
5. It's absolutely not going to happen, but honestly, a few wholly new Pokemon to fill out the tiny amount Kalos introduced wouldn't hurt. We're not playing on GBAs anymore, guys. Patching in updates in the middle of a generation is totally fine.
Oh, fuck it, they probably did something asinine in the implementation that would make it totally impossible. This is GF we're talking about, after all.