I feel as though the island should be able to have just farming plots or greenhouses that can hold like 4 farming plots in it. This is because right now I'm currently using just my farming plots right now and not my housing plots. Also, I feel like we need a worker that's next to the merchant that gives you items, and that worker should plant seeds based on how many seeds you give him. This will cut the amount of time compared to just clicking one plot.
I think allowing more farming plots would create troubles for the price of farm products. People with multiple islands could break the market. Maybe it could be balanced with the cost of the greenhouse buildings, hard to say.
That aside, I'm all up for any mechanic that reduces clicking for island tasks. Having a npc who could plant, water and harvest ur fields would be awsome. Same with one npc taskmaster that gives and collect ur journals to/from laborers. Add a channeling time for it, so not to making having islands even more profitable. And all the people with carpal tunnel or other wrist problems will be thankfull.
If people thinks islands would be too easy due to this change, or if there is people who likes the gameplay style of farming, you could add a small silver cost to using those npc or even require a manager building to be built to keep those npc's. This things would nerf islands a little and allow anyone who likes farming to still do it for a little extra benefit.