Above everything else, sidequests, optional content, questlines you can find at your own pace and complete independently from the main story. More exploration in general which would tie into this.
Also pretty high up there is difficulty modes, pretty damn basic stuff.
Also progression for your character, not just Pokemon. I don't mean your character gets to do battle or any stupid shit like that, but rather you get to work on increasing your proficiency with various skills, such as capturing Pokemon, breeding, training, trading, fishing, and so on and so forth, each little facet of the game treated as its own progression track with rewards and unique quests tied to it, maybe even some crazier "professions" like photographer which would turn the game into a Pokemon Snap sequel if you so desire. This is very popular in modern RPGs and it would go well with Pokemon. A good comparison on the 3DS is Fantasy Life, where you could pursue aspects of the game and focus on them if you wish, or do a little bit of everything.
