In my ideal world, people would be of varying sizes, because that's how Mother Nature made us. Some of us would be painfully thin, some would be medium weight and some would (gasp!) even have BMIs over 25.
However, in my ideal world, there would probably be much less obesity and morbid obesity. Many fewer eating disorders too: less anorexia, bulimia and obsessive overeating.
Parents would respect their children's need to eat or to stop eating. There would be no "clean your plate, kids are starving in Africa!" Nor would there be policing of the child's food intake (see this horrific report).
Parents would not use food for punishment or to show love.
Everyone would take phys ed at school right up until high school graduation. However, in regular phys ed classes, the accent would be on participation and movement. Gone would be the days of choosing teams according to talent. No one would always be chosen last. Gifted athletes would have "enriched" phys ed classes but ALL students would be encouraged to enjoy physical activity, no matter what their level of athleticism. This is a real bugaboo for me. I was not athletic, felt belittled (pardon the pun) and shunned in gym class and thus stopped taking gym the minute it became possible. I wouldn't force kids to take music all through school unless they really loved it, but phys ed--that's learning to live a healthy life. Everyone deserves phys ed all through school. People would make exercise an integral part of their lives throughout their lives.
Foods would not be doctored. Low-calorie versions of real food would be ignored to death. Artificial sweeteners would have the same fate.
Inasmuch as possible, we would eat locally grown food. Local, small-scale farming operations using fewer/no pesticides would be encouraged. Animals would never be fed antibiotics to fatten them up.
There would be massive investments in public transit. Taking public transit automatically means walking more. Just to give you an example, on Wednesday I exceeded 10,000 steps. I would have been at around 4,000 if I hadn't taken the subway to work. I know it's not always possible to use, but we need MORE public transit, not less.
Everyone would learn to meditate. I'm not advocating we all become Buddhists. Meditation is a valid, non-denominational, no-cost tool to help ease stress, lower high blood pressure and just feel happier. It works.
People would lose their fear of food as they learned to give it its proper place: not something evil nor something that solves all of life's problems.
And thanks to everything I've mentioned above, diets would become unnecessary.