Probably the biggest environmental win would be to make arcologies: ecologically integrated cities.
> Arcology, a portmanteau of "architecture" and "ecology",[2] is a field of creating architectural design principles for very densely populated and ecologically low-impact human habitats.
> The term was coined in 1969 by architect Paolo Soleri, who believed that a completed arcology would provide space for a variety of residential, commercial, and agricultural facilities while minimizing individual human environmental impact. These structures have been largely hypothetical, as no arcology, even one envisioned by Soleri himself, has yet been built.
https://en.wikipedia.org/wiki/Arcology
You would include the "Living Machines" designed-ecosystem technology of John Todd, et. al. to process waste and produce food and (some) medicines on site.
https://www.toddecological.com/
I dunno if you could make a startup out of it though.