I'm contemplating the density question, vis-a-vis issues of per capita environmental impact. I ran across an interesting graphic the other day.
Here's what I starting thinking about, in seeing this. The "Paris" version, above, is the densest – so imagine the world's population living in that space. That would be one messed up ecosystem, there on the Mississippi delta. The impact would be, essentially, total. But think of this: the rest of the world would be empty of people. Maybe there would be some agriculture – this sort of graphic doesn't say how putting everyone in one city would see how their resource needs were taken care of, how they would be fed, etc. But let's imagine a best-case scenario, with all the people living in this giant megalopolis in the Mississippi delta, and then a bunch of sustainable automated farms and mines feeding it. Hmm… kind of science-fictiony. And I don't want to try too hard here. My only thought … my main point… is that this mega-city's impact would be huge, but the rest of the planet would have much, much lower impact. That seems to lead to the ability to imagine the Earth much more sustainably carrying its current population. QED Density is a good idea.
Just more random thinking.