That's not necessarily what the line in the prophecy has to mean. Really: world=earth=Gaea. (Most of us figured this out even before it was stated in the book.) So the question of who's going to die, if anyone, is not 'storm or fire', but 'an oath to keep with a final breath'. That's not to say, of course, that Leo is definitely going to die. It may very well not even be any of the seven to make said oath, and who's to say that the one who makes the oath isn't going to live in the end? 'Final breath' may mean last mortal or immortal breath (*cough* Calypso *cough*). And if you're still skeptical that it could be someone outside the prophecy, remember that, at one point, we were all fairly certain that Percy was the hero of the Great Prophecy. SOMETIMES, NO ONE DIES, OKAY? DO NOT QUESTION MY UNBRIDLED OPTIMISM. IT IS DEARLY NEEDED.