Self-contained, solar powered domestic toilet wastewater treatment plants.
Pretty amazing tech using relatively simple chemical reactions and low power requirements. It’s a bit expensive for global use right now, but that’ll change with economies of scale. Read more about it here. I think if you could couple this with a Kamen water purifier (which distills pure water from any source) as well as some of the Star Wars inspired “Moisture Vaporator” tech that’s coming online, you’d have a real winner. Get free water from the air. Use it to flush your toilet. Use resultant effluent for fertilizing crops or filter for flushing or even consumption. A neat cycle, all powered by the sun, that significantly reduces resource usage and also decentralizes utility use, making homes more self-sufficient and less dependent on the grid. Neat all around.
As funny as it is to see Bill Gates with imitation poo, I think the implications of such a “dirty tech” as a reinvented toilet represents a pretty noble technological ideal.