I have tons of ideas, but at the moment, I think work on the sound design would be nice. Partially from the TV, how the sound fades when you move away from it. It seems to cut out suddenly when you turn a corner, and more incremental fading would be nice for immersion.
But mostly I think having rain sounds when you're inside would be huge for immersion. Kind of like the Nevada Skies mod from Fallout New Vegas. When you're inside you can hear rain on the roof like you would in real life, and it makes the game so comfy in that moment. I would love improvements to the sound design such as these.