I'm not patient clever great at character creation, so I always base my farmer off another fictional character that I like and then use their personality traits to set the gameplay style for that run. Then that, in turn, leads me to certain design choices and I just kind of follow the natural theming form there.
For instance, one of my farmers whose farm I've posted on here (Jones Farm) is Jughead from Archie Comics. He values friendship and food above everything else. So socializing (something I don't normally focus too much on tbh), helping his friends, and being able to cook all the delicious recipes they send him were priorities. That meant little picnic or dining areas around the farm. Plenty of benches to sit and hang out with friends on. Enough farming space for every in season fruit or veggie, chickens/ducks for eggs, goats for milk, fishing and fish ponds/crab pots to make sure he had all the necessary ingredients on hand. Beautifying areas around town for his community (and giving most of them dining areas).
There's also a practical element. I don't want to have to run clear across the map to pet my animals on a mining/fishing day, so I either want them close or to have a clear, easy path to run to the coop/barn. I don't mind traveling further to water/check/harvest crops or chop wood though.
The shape of the farm helps. I favor Hill-Top and Wilderness, which have a lot of naturally defined spaces that tend to guide where everything goes. I'm currently using both the Four Corners and Beach farms for the first time. Again, the shape of the farms guided the design. For Four Corners I went with a four seasons theme. Each corner is themed on a season and that surprisingly split up 'uses' even more than the built-in function of each corner (foraging corner, mini quarry, lake, farmhouse) did. The farmer on the beach farm is based on Leon Kennedy from the Resident Evil games and I took a lot of inspiration from the S.D. Perry novelizations (describing his drive to Raccoon; the fields, the woods, etc) that, combined with the shape of the farm, made it really easy to decide what would go where.