"Special Diet App"
Lots of people are on special diets e.g. my Mum is on a "low salt diet" but what does that really mean and what ingredients can she cook with, what tasty recipes can she try (food without salt can often taste bland) what foods should she avoid?
I think lots of people would find such an app helpful. They can use the app when out shopping or planning meals. Other examples include low fat diets, high fibre diet, low sugar diets, low potassium diets, iron rich diets, low protein or high protein diets etc. You can apply all the necessary caveats. A smaller scale idea would be "dish of the week" for each of the special diets to inject some interest and introduce new recipes.