Sort of an AHC and a WI, make Hadrada have both de facto control and de jure control of the Kingdom of England by 1070. Rules: POD has to be after 1064. No ASB. William must be stripped of all future intentions to get the throne of England, maybe he dies, maybe he is defeated in battle, whatever you chose, but he must not try to get the throne after 1070.
Now the WI part: According to whatever your solution to the challenge is, what would be the effects on England? What would it be like by 1500? Et cetera et cetera...