What if, when Henry II of England makes his daughter Matilda his heir, he marries her to his nephew Stephen, making them his heirs jointly instead?
Yes, I know Stephen was married - though I'm not sure when the marriage took place - but she could easily die in childbirth, etc. I could see this staving off civil war, but if it works out long term probably depends on whether or not Stephen's prior marriage occurs and if he has sons by his first wife as well as with Matilda. The half-brothers might well come to blows.
But of course, one major upshot is, no Plantagenets, which changes English history completely.