POD: William the Conqueror beats Hardrada to England in 1066. William's army is defeated at Hastings, but Hardrada arrives two weeks later, and King Harold marches north quickly, where he is defeated at Stamford Bridge (or nearby). Just to throw in a twist, Tostig (Harold's brother, who was with Hardrada) dies in battle, so Hardrada claims the throne of England.
Politically, we'd probably see a Danish England for some time. But would that endure, or would England later break away? For that matter, how would French history play out without significant English involvement?
I'm also interested in the cultural consequences. What might England look like with a Scandinavian aristocracy instead of a French one?