It's good and reasonable from your part to not set any high expectations. It makes things easier along the way.
It's good to see you motivated too. On my part, the music is the hardest because I am not a composer and it's very hard to find a royaly free music that stands out of the lot that will easily make your game recognisable. I take Super Mario Bros and Eagle Island which both have easy to recognise music theme and that, just the first 2-4 seconds of the music. Then there's Skyrim which has a more dramatic and cinematic music which makes us want to dive in the adventure. So for me, it's a real challenge.
For sprites and interface, everything is about color and well matching things together. Learning to use gradients and shadows AND learning to NOT use them at times. There's an expression we use here in Quebec, translated from french to english it says: "Too much is like not enough". Meaning both are not good. I learned that in my passed years in game making. Having a balance between well merged colors for smooth detailed scenes while keeping the characters easy to follow and not loose them in your background. For interfaces, a lot of people keep it too simple or overly complicated. When I say complicated is too much colors and sprites for a choose-to-sell-your-stuff screen. Learning to design an interface that keeps the information handy well visible and easy to manage while adding a style that fits the game is a nice challenge. Adding transitions, like fading your background and pausing the game add to the effects.
Other things people tend to forget in games is life. Life in a game, and I really love Eagle Island as a pixel retro style game as an example. They added tiny frogs, rabbits, moles, dandelions and moving grass along with balancing lanterns and vines that sway when you pass by them. In my game, Lost Explorer 2, It was a boring game initially and I had to add a little something. Even today, I think I lack a heck of a lot in that game, that's why I want a third one. But, the fact of adding fire using the particle system, made things a bit less boring. And in dark levels, adding light that actually moves added life to the fire itself. Today, I would change a thing or 2, but the fact is, simple things add life to games which makes the screen more appealing.
In your fishing game, simple water waves and small shore lines animation, some grass moving here and there randomly, simulating the wind, a sun that moves during the day where you can see the shadows change and colors adapt to the proper time, little bugs and insects hovering above water like dragon fly vs some ordinary flies in more damp places. Adding color gradients from blue to green in places where moss and mold can form. This details, while not of capital importance will bring a heck of a lot of life to the game. Then, there's the opposite, too much animation will hinder the game because fishing is supposed to be restfull, stress free, so you want to keep things living while peacefull and the menus and interfaces should breath the same ambiance than your game.
I really love the idea of your starting lake and cabin/house. That's a very neat idea. In Skyrim, one of the first towns you meet when finishing the initial escape scene is Riverwood, a peacefull watermill river-side little town with gentle common people. It's not a player home, but you still feel home because it's the first place you meet. When you create your initial map, people will feel home immediately and tend to come back there because it feels good to come back just for the sake of relaxing and managing our inventory and stuff. I'm bursting out a lot here, but I am pleased to see you excited about your game.
I'm eager to see the advancement of this project. Great work and stay motivated