> Either rooftop solar, or green roofs everywhere

If you want a dense city, you want green roofs - with access. Land in a city is too valuable to waste on solar - stick that stuff out in the desert where nobody wants to be.

> Cut-and-cover highway tunnels

Take an existing dense city like Manhattan and build a new platform above the roads. Existing ground floor becomes basement, new platforms become park/pavements/plazas rather than for cars and trucks. Cities rise anyway - look at down town Seatle for a high speed example, but if you go to old cities in the UK, say Cambridge, and look at buildings 1000 years old, they're a significant drop down from today's "ground level".