Your challenge, should you choose to accept it, is to create a scenario where Henry the Young King and Geoffrey II, Duke of Brittany, become King of England, et al, one after the other, instead of their more famous siblings, Richard and John.
Simple: Henry the Young King survives, outlives his father, and then sees his brother, the new Duke of Aquitaine, off to Crusade. He dies, and the King of England wars against the Duke of Brittany for Aquitaine. The King dies, and now Geoffrey is King.
Would Richard go gallivanting off on crusade if he's only Duke of Aquitaine? Wouldn't he fear his brothers would prey on his territories in his absence, or the French (if circumstances have changed such that Philip doesn't go on crusade)?
I imagined, in this scenario, that Richard leaves his Duchy in the hands of either Eleanor, who in theory should be safe from her sons, or his wife, whoever she is in this scenario. I also can't imagine the French not joining the crusade, but that's just me.
Fair enough, I can't say I know much about the French at this time.
In a scenario where all his elder brothers are alive what's John doing? There was obviously that proposal to set him up in Ireland, but I'm not sure how feasible that was.
As for John, that depends on how his life falls. If he can get something going in Ireland, he'll spend the rest of his life fighting for that. If not, he'll probably push for a consummation for his marriage to Isabella of Gloucester, or maybe an annulment so he can marry elsewhere. Or, he might be married to another heiress. Depends on how history develops with a surviving Henry the Young King.