As an addendum to the Open Borders discussion...
In an effort to continue to fiddle with diplomacy using my pathetic, XML-only modding skills, I decided to make Defensive Pacts appear earlier, with Writing. I had two reasons for this. First of all, I wanted to have more lopsided wars, which might cause more civs to fall. Second, I wanted the friendly civs, mainly India and Egypt, to be more likely to be drawn into wars. I decided to play as a fairly isolationist China, so I could watch.
The idea, on one playthrough, appears to be a failure. Every Civ still survives, into the 19th century, and I haven't noticed any significant changes in territory.