I think the biggest problem with our agricultural industry is commoditization of vegetables. We need 'celebrity/hipster' vegetable strains/growers that command significantly higher prices. If you grow vegetables that are twice as nutritious as found in the grocery store, you'd have a hard time finding a market at twice the price. Nutritional density of our vegetables has dropped something like 10% in the last 30 years as a result of agroindustrial farming methods.
Megacities are certainly problematic for the 'local' part. A city like Chicago that's more-or-less surrounded by farmland could be more plausible. Microgreens with imported seeds could be an interesting option: locally grown, fresh, short growing cycle of ~14 days, one warehouse full of growlights and you could provide greens for thousands year round.
Perennial grain research, such as Salish Blue 'Wheat', is also pretty exciting - would greatly simplify grain farming if the plant lived many years.