Regardless of specific victory conditions, as long as the top three players win I imagine some dynamics will change for the worse. As is, you often see two big players going for each other. What if the two biggest guys on the block now say "screw this" and together start gunning for everyone else?
Would perhaps make the game a bit shorter, which is good
But if the most powerful nations never fight each other, is the win really a valid test of player ability/nation power?