So I in fact became interested in alternate history only recently--though true history is my forte--after reading Ken Follett's Pillars of the Earth and wondering what might have happened had William Adelin, heir to the throne of England, never died in the White Ship disaster in 1120.
The shipwreck left King Henry without a legitimate son. Though his barons and earls swore loyalty to his only other legitimate child, Matilda, after his death many reneged on their promises and ended up supporting his sister's son Stephen of Blois, who was a bit older, had been close to the king, and--most importantly for the time--was a man.
The twenty years of Anarchy--as the period is called--that followed resulted in widespread low-key devastation, looting, and pillaging of most of southern England. The Welsh invaded and looted around Gloucestershire and the marches, and the Scots broke free of Anglo-Norman hegemony and invaded Northumbria. England overall was weakened, and earls and local barons gained power at the king's expense.
Now, Henry ruled as a strong king but was a bit unpopular with many of his barons and earls--mostly those who owned land on both sides of the Channel--as well as with the Church. He reformed and reorganized Norman England into a more efficient machine, used existing English legal and governmental institutions to aid his rule and ingratiate himself with the commoners, and was even alleged to have learned English.
What I see is William Adelin reigning as William III, and for the most part continuing his father's policies. Though I've written stories and stuff before, nothing I've ever touched has been alternate history...I wouldn't know the first thing about forming a timeline and predicting likely events. Still, I know that no Plantagenet kings would lead to a much quicker absorption of English as a language into the upper strata of society. England and France would both honestly be in much stronger positions throughout history without the wastes of the Angevin wars of aggression and Philip II Augustus's required reconquest.
Because the POD probably doesn't butterfly the general occurrence of the Crusades, maybe the English and French are more successful there?
Thoughts, ideas, help with writing, maybe?