It would be awesome if there was an option to make a custom deck when starting a battle. That way you don't need to keep changing your pre made decks to fit the battle/monster type.
I would be cool if you could level up the workers, or have them get new skills like shading or something as they got more experience. The way it is now there's nothing to really work toward, so achievements could make it more interesting as well, like "Drew A Perfect Circle" if a character gets 60 skill.
I would like if the market had a complete sell all button. Like, you'd sell your whole catch, instead of all of one species. I also think that the penguin should have a mute button, because when I was watching the movie his talk bubble was getting in the way. And I've always wanted, in games like this, a button that will show you all of the things you can afford, so you're not looking in every category for anything under 1000$ or whatever. It would show everything in your price range. (I just started, so I'm not sure if these are valid, but here you go).