Wouldn't it be fun if you dropped a god into your campaign world with his own agenda and just ran it as a subplot? That's what I'm going to be doing.
In my game-world, Tharg is a kind of Norse god of battle equivalent; the god of war. He's going to pop up somewhere in the lands to the south and start calling those Free People with fire in the their veins to him to form a vast army, and then he's going to attempt to crush the Great Hoarde into dust.
The players will learn about this as rumours in taverns and so on - "a man is gathering an army to him in the deep south!", "there is a rumour that Tharg himself is walking the land, preparing for a Great War" until one day they bump into a man who works for a man who works for someone else who is claims to be working for Tharg himself, bearing a message: Stand beside the God of War and make bloody battle with him against his enemies.
This will be completely and utterly optional. Its likely the party will have too many quests and obligations to do this, but I don't mind if they decide to join his army and swear fealty to him to participate in an epic battle featuring hundreds of thousands of warriors.
If they don't, they will hear how the great war goes as it happens. They learn through rumour whether the great lord of war defeats his equal and opposite in single combat while his huge army battles the great hoard in a valley stained with blood. I'm sure it will add spice to the campaign and will help the players realise they are not the centre of everything that happens...