A quest log of sorts would be useful. It could list the main story objective incase someone gets lost or stuck (I remember my first time back in Diamond I was often unsure where I needed to go next because the region wasn't "linear") and update it as you progress of course.
It could also list all the sidequests you are given, no matter how minor, once you have spoken to the person at least once. Some such as just buying the berries from the girl on Route 5 daily, or rematching Cynthia, getting a fossil from the fossil guy, beating Black Tower/White Tree etc. Just to give some direction, because I often find myself having to sit with the laptop open to find out all the things I can do daily/weekly/post-game/etc because even though I've already spoken to the person times before, I just can't remember everything when there's that many little things to do and collect.
I'll probably have more ideas once I think about it more, but the last few days playing B2W2 and not being able to know what stuff I can without either trawling the entire region again or having to look it up online make me think that a Journal that records objectives, quests, missions, whatever would be really handy and useful.