I'm having fun playing a house with this because it's something I've never done before in the Sims. But there's so many things that need tweaked or added. If they ever do add more to it, what would you like to see?
Solar Panels. But with limited use if you are off the grid, I don't want my Sim to be able to use a computer without going to the library. Or watch tv. But it would be nice to be able to use the older looking radios. And, if they can't heat water on the stove for a hot bath, maybe the solar panel could allow hot showers or baths (with an imaginary water heater I suppose). If nothing else, being able to use tablets and my Sim's phone would make a whole lot more sense if I actually had a way to charge them. On the grid, they'd be great for lowering bills like they did in Sims 2.
Usable Well. Since there are sinks, this isn't completely necessary, but it would be fun. If your Sim could have the option to get a drink or their laundry water from it. Perhaps a cistern that catches rain water instead? Also more sink options would be nice.
More cooking options. There are plenty of things that can be cooked over fire.
Canning. I'm not normally big into gardening, but it seemed appropriate in an off the grid house. So my Sim has a garden full of things that could be canned or made into soups. And this is a feature I could see people using off or on the grid. I used to use it in Sims 3 just to make a pantry that looked good. In this same vein, wine making could be nice.