One that would interest me greatly is:
What is the impact of diet on frequency of visits to the healthcare system for illness (as distinct from accident)?
Within that, what is the relationship in respect of:
vitamin C intake;
raw fruit and vegetable intake?
I strongly suspect that incidence of illness and visits to the medical system could be reduced by more than half if adults took at least two 5 gram doses of vitamin C per day (and children smaller amounts prorated on body weight).
I strongly suspect that free supply of vit C to everyone would be the cheapest and safest thing the medical system could do for all people. Procured in bulk it could be a very cost effective option.
I would love to see that study done at scale (at least 10,000 people over at least 2 years).