I'd imagine as suggested Hadrada becomes king with Tostig as regent. By 1075, he'll probably be dead. Tostig claims throne, major rebellions with Aetheling as the figurehead (whether he likes it or not!). Half-hearted Norwegian efforts to reclaim throne, leading to a defeat on English soil. Edgar crowned king. However what kind of king he would be is interesting and very hard to figure. Would he be a new Alfred, throwing the enemy into the sea or just a puppet for the aristocracy to fight over.
Interesting tangent, if William dies on campaign, what effect will it have on Norman power? Most of his army were French/Breton auxiliaries working 'in hand' until suitable spoils could be won in England, Id imagine the survivors would be pissed off. Civil war for the Dukedom is very possible and wont help matters. Will the eventual winner look to England again or will Normandy be severly weakened? Will the Normans be able spread their influence into the Mediterranean and Holy Lands?
Without a cross-channel dynasty I imagine Anglo-French relations will be VERY different. Will the English conquer the British Isles sooner? Will Norman power focused on the continent weaken the French kings' efforts to centralise?