What if you do a Prometheus thing? He has taken a powerful secret and wants the rest of the world to have it. Or maybe reverse the story, and make the material plane have something powerful that he needs to steal and share with the elemental plane to start a revolution. Alternatively, he wants to take back the secret of fire from mortals, to bring the power of fire back to its stone-age reverence as a thing of mystery and divinity.
I like it. What if the elemental was Prometheus himself, having returned to the material plane, expecting to be worshipped as a god. He comes back expecting parades in his honour for being the savior of mortal-kind, but finds that he is forgotten about and barely a myth now. He has been changed from his years in the realm of fire, he paid a terrible price for stealing the secrets of fire, he is more elemental than man now. A human that is constantly on fire, but never consumed, to quote him "after a millennium, the pain simply becomes an environment". I like the imagery.
Is he taking the secret of fire back from humanity because?:
1) He has only been released on condition that he does this
2) They were ingrates who refused to honour his meglomania and sacrifice
3) He wants to rule the world and now only those with his blessing will have the gift of fire
4) He wants to plunge the world into an everlasting winter, because he knows the elemental prince of fire is coming and he thinks it is the only way to save the world
This works best in a Warhammer style magic system where you have the different Winds of Magic. He could be gaining control of the Wind of Fire. If he succeeds, nobody else can call on elemental flame. No fireballs for anybody. That could be a cool quirk for a game, half way through the adventure, every mage in the world loses the ability to do fire magic. What does this mean for religions like Pelor? Does Prometheus claiming dominion over all fire wither the reach of the sun god? While Pelor is at his weakest, who will make a move on him?
<P.S. Oooh, just had a cool idea (if I say so myself). What if you go with the Prometheus thing, but he turns up and wants to give everybody in the world the secret of fire. Imagine a world where everybody can make a warlock style pact with him channel power from him. Every farmer and labourer can cast Burning Hands, Fireball, Scorching Ray etc. Often not with great control. It seems like an amazing gift, but imagine what that does the the political structures, armies and faiths. Everybody knows its a bad idea, but if your neighboring kingdom has 1000+ guys who can fireball you to death, you better get in on that act yourself. Eventually, the whole world will burn, is that what he wanted all along?>