On the note of Bahamut dying right off the bat before the game starts, as Anonymoose said, it could have been planned for us adventurers to have to go on a "revive Bahamut" main story quest line at some point in the future to battle whatever awesome force of power we've yet to hear about. Also, as we don't really know anything about Primals, we can't even be sure if they can be killed. Maybe by "dying" they only go into a petrified slumber as we see Bahamut as now.

My theory was that Garleans were using "Eikons" to power some of their machinery. (Which might explain why we experience the Echo in the first place. Our primal - possibly Siren, hence 'echo' - may be captured by the Garleans and is calling out to players to set her free).
I like this idea, it's heavily reminiscent of the whole Espers and Magitek Factory with the Gestahlian Empire in FFVI and I always love throwbacks to FFVI. =D