I think it would be really neat to see the AI's themselves choose their own alliances over time. Maybe for the first month of launch the Vex and Fallen could be super good bros, but also as the game progresses on with its life span those two would split apart and bond new alliances, and maybe even rivalries. I feel like this would really support the idea of Destiny being a "living breathing world." I mean, who wouldn't want to be plowing through the sand dunes of Mars on a stolen Pike, only to catch some most excellent air off of a jump and land into an encounter of some Cabal putting the hurt on some Fallen invaders? Pretty unforgettable stuff right there.