I think something that would be interesting to add is aesthetic builds. I see people focusing on not only making their farms profitable, but also beautiful. The designs they come up with are great, but I think adding things like outside furniture and decks would be a good idea. Heck, having some already in-game furniture made to be placed outside might make this a simple suggest to execute. It would give people interested in this sort of designing an aesthetically pleasing look to aim for on their farms. For example, I could take some normally indoor benches, tables and plants and make a nice looking sitting area in some extra space I couldn't find a use for. It would also be a good way to prevent rocks/grass/sticks/seeds from spawning in an area you don't want them spawning in. Past that, I would just say more furniture in general would be nice anyway. I have been having so much fun playing Stardew Valley since I learned about it from my little bro and I am glad to have picked it up! It is nice having a game that reminds me so much of the Harvest Moon games I grew up with, thank you ConcernedApe!
I agree, outdoor furniture would be great. A little patio with a table and chairs would look great. Also things for the kids to play on so they can go out side, which could either be bought, or crafted.
I hadn't even considered things for the children in the game to play on, that is a very good idea as well! That would be just the thing for a family oriented farm. And as another idea for the patio, maybe an umbrella as well.