I’d be really interested a shopping list app.
The first iteration could be really simple: My wife and I enter what we need to shop in the same list, and we tick off as we go through the general store.
The next iteration could be stuff like assigning what to buy to each one of us, and categorize things.
Then, you could simplify addition of goods that you buy frequently, and to re-add a group of stuff. Then, you could integrate recipes.
Then, at some point, I hope general stores will publish Linked Data so that you can connect things on the shopping list to concrete products. Then, they might publish where in the store you can find things, so that the app can optimize your walk through the store. Why would they do that? Because it is the only way they can survive in the competition from online stores…