Ability to set more flexible macro goals
right now, Macronutrient goals can only be set as a percentage ratio, which dynamically scales with energy expenditure. So, if I do 500 calories worth of running, my macro targets for P,F,&C ALL go up.
however generally the way my diet works (and I think this is the case for many people involved in sports) is that I have a target goal for Protein in grams rather than a percentage. So, say 130g protein. Then I want any "extra" calories I burn from exercise to come from carbohydrates. So if I do 500 calories of running I want my protein goal to stay the same and just get the extra cals from carbs.
there are other cases as well - some folks on ketogenic diets may want to have a "maximum" carb intake set at 20g which doesn't scale with activity. people may want to set a minimum fat intake, or to have all of their "extra" calories burned by exercise come from fats instead of carbs.
this is something I have been looking for in a diet app for a long time. A more robust way to define rules for macronutrient targets.