I'd think that by the time we are down to a billion people, energy will essentially be free and carbonless. Cities will be hyper dense. Most agriculture will take place in insanely efficient greenhouse conditions. Getting there will probably be a fucking blood bath and we will spend hundreds of years cleaning up the mess we have made.
The question, then, is whether you see a Jetsons or Road Warrior future. I think it depends on how we get there. It's been my experience that if you want to see what the world looks like in fifty years, look to Japan. Japan is hyper-dense, 60% of its agriculture is shipped in from abroad and getting there was a fucking blood bath. Tarkovsky got permission to shoot in Tokyo for Solaris. There's a whole bunch of hand-wavey shit about why there's a five-minute scene of driving around Tokyo in that film... but at the end of the day, the State wanted their money's worth and Tokyo was the goddamn future if you were Soviet in 1971.