I would like the ability to terraform your farm (Like Animal Crossing) or the ability to customize your farm before you start, that way the game has even more replay value.
A random thing I would like is bathrooms in all the houses, I don't know why I just thought it is odd that all the houses didn't have bathrooms lol.
The biggest feature I would like is an expanded map, more things to do, explore and find. But that is more of a dream, I like the map as it is at the moment though after 233 hours I did wish the map was bigger.
Also the ability to play billiards on the pool table, then if you win 10 times against other villagers or something you could win it and put the pool table in your house. Also, Darts would be cool, or anymore mini-games that I could play to give me another 100 hours of playtime.
Hedges, pet events and multiplayer minigames (such as you can verse other players in blackjack at Sandy's) would be amazing.
But at the moment anything little or small added to Stardew Valley is amazing and the game is pretty much perfect.