I have a simple suggestion which would make Minecraft more realistic. The water that is in the snow/ice biome, especially the water under ice, should directly harm the player the minute he or she touches it, which means that you can't just break the ice, swim in the water, and survive for more than a few seconds. I suggest this because I believe swimming in extremely cold water in reality is extremely dangerous and may just literally freeze you to death in a short amount of time.
It also doesn't make sense to add unless cold weather in general can injure player unless protected somehow, and that seems like a hassle also.