A nutrition tracker where you can enter what foods you eat into a small database. And then when you eat meals you can check those foods off in order to calculate all of the nutrition facts consumption per day. And it could be expanded even further by adding graphs and reports such as Weekly, Monthly, or Yearly.
Could track Calories, Vitamins, Minerals, and other specific nutrition stats. Most nutrition apps I've seen only track Calories... Or don't have accurate nutrition applied to specific foods as it is generic. Letting the user add the food as a item in a small database would give the user more control of how the stats and reports are calculated.
Could be helpful for some to see their intake and then figure out ways to change it to become healthier.
Isn't this literally what Waistline is for Android? You create your own local food database (which you can automatically fetch info from Open Food Facts or USDA if desired, but not required) which lets you put in as many nutriments to track as you wish, all with graphs and information with different timelines.
No clue if there's anything like this for desktop.
Also, export the graphs/reports to PDF, or something easy to pass to a doctor.
Maybe a dev can take inspiration from the Yuka app. To their credit, they have put together a great database for scanning foods and comparing ingredients, as well as offering understand of the possible risks of those ingredients.