Be nice if more food was on the market. Maybe add a caravan that specializes in trading Carrots, Apples, Brahmin Meat and so on. Would really help out. I've never really tried to make an NPC with that much scripting behind it though, so I don't know if you could just copy a template or what.
Another thought would be something like a food fabricator. You could find them in the diners of vaults, and they would work similarly to the Sierra Madre vending machines. The most likely currency for a system like that would probably be radroach meat. It's sort of easy to come by, it can be reasoned lore wise why it would operate on that and so on. The pay-out would be various pre-war foods.
Also, going way back, yeah. The first time you ever see Xander Root and Broc Flower in Fallout is up in Oregon in the Arefu settlement. They're most likely mutant plants, so they could go anywhere almost. Probably not some place like the Pitt.
If you really wanted to add a farm to Megaton, the most likely, and easiest crop for them would probably be Mushrooms. Just add a door to a damp underground area, like possibly a Drain Pipe that was exposed and broken open when the crater formed. Put various mushrooms around in there and that should work. It would give Megaton some reason to exist, outside of possibly also distilleries. That would be kind of an interesting little cash-in quest. Someone who pays you for empty bottles.
At any rate, DC has logic and design problems all over. But it does sort of make sense if you read too much into it. It's not about any one settlement self sustaining, it's about the caravans that supplement and supply everyone. Arefu sells the brahmin meat, Rivet City the vegetables, Megaton would be a natural fit for mushrooms and liquor. Then there are the smaller places that obviously only buy, like Tenpenny Tower, Temple of the Union, and so on.
It's been a long time since I posted anything, none of this probably makes sense or matters at this point lol