I'd love for ponds and mini lakes to freeze over in winter and have mini fishing holes in the ice where you can still fish for pond and river fish. Then we could have ice skating mechanics or something cute haha.
Just read op and all the responses and I love it when people have cool ideas and input for the game.
edit: going back to the colour of the river - I'd love a more green/blue or purple/blue for the water anyway but I wouldn't want items near the river affecting the colour of it, maybe affecting the types of bug or fish that spawn though.
I love this idea. It would add so much depth to AC. I can just imagine rivers being a murky color for a change, or freezing over during winters--allowing your character walk over them or dry up enough that you can hop over them in during really hot weather. If not in your own island then at least mystery islands where certain water types make catching certain fish easier. It would make venturing a lot more fun.