I would love a farming EP. But it needs to fit with the Sims 4 works, and it's really a game about everyday life. It cannot really be about big industrial farms with huge fields with big machinery and barns filled with animals. If it's too big, the game cannot handle it anyway. It would not be an industry, it has to be something you can do on your own lot, kinda like a side hustle, or something you do just to feed your own family.
So I'm thinking if they do it, it would be more simple. Like a chicken coop is easy enough, kinda like the bees in Season or Insects in EcoLifestyle, similar to what they gave us in Sims 3. You don't want the chickens to be family members and control them anyway, so just an object with interactions like feed, clean, play with the babies and scoop the eggs. Maybe, with the same object, we could have access to different types of chickens, or even different types of bird ? Like geese and so on.
You won't be able to have herds either. There is already a limit where only 20 sims can visit a lot at the same time. So if you have a family, and you add maybe a cat/dog, you invite a friend over... not much wiggle room. So, instead, since it would be just for the family, I'm thinking like one goat, one cow or maybe one sheep. Either it's a family member and it's like cats and dogs, or it's more an object kind of thing, like the chickens I was talking about. It would be better not to be an object too much, so you could build a space for it yourself, like a barn and/or a pen. It would be really fun if you could see them wander and graze. They could have a tendency to "escape" and eat what they are not supposed to, forcing you to make sure their pen is secured :)
I once saw on the forum someone say that Sims wouldn't allow for the killing of animals in game, even off-screen. Like you wouldn't be able to send animals to be killed to get the meat. I know this is how it works in real life, but the franchise would probably face backlash. I feel like it's true. Therefore, no pig. You could only have animals that produce something else, like eggs, milk/cheese/butter... This, with of course more plants, would mean new recipes. You could have an object where you put in the milk and after a bit you get either cheese or butter. And if you have sheep, then you get wool, but I'm not sure what you could do with it since they are already doing a Knitting Stuff Pack...
As for the plants, maybe a new type of plants : grain ? And you can have a mill object to make flour ? And maybe brew "beer" or something. Also I miss being able to make wine like in Sims 3, where you could store it in caves and it would gain in quality !
Basically, I feel this EP could be more about life on the countryside than about farming as a living. It allows you to add plenty of little elements instead of trying to implement a big system. I'm thinking like making jams, canning... and maybe something with fish cause they always go bad, so maybe salt them or something ? And, of course, a farmer's market. Either like the festivals in City Living, so a set place and a recurring time, where you can sell and buy, or just a Community lot. And add a way to make the selling easier from a selling table : instead of selling each fruit/vegetable/flower individually, it would be nice to sell a bunch at a time. You could sell them by the basket !
I'm sure there's plenty more to add !