Depending on my quests and whatever recipes I'm trying to make, it can get confusing when I'm out and about trying to remember all the various ingredients I need to buy/harvest.
A shopping list function would let you drag an icon of an item you need onto the list and indicate how many you need, then you could reference your list when you're out on the town.
Ideally, you could only edit your shopping list from inside your home, since a) that's another benefit of homeownership and b) everyone stores stuff in our homes and can only do an accurate count of what we have in inventory+house when we're there.
I think most of the house models have a fridge, mine has a graphic that looks like a note is on there, would be a great place to make it interactive and let you link to "add" things to your shopping list.
Just a thought! :)