Henry II of England carefully constructed a scheme for inheritance for his sons. He crowned his oldest surviving (at that point) son Henry to designate him as heir.
The thing was as a crowned king, Henry the Young king was expected to show generosity and have an impressive court. But he had no land of his own and his allowance from his father was similar to the income his grandfather, Geoffrey count of Anjou, had to spend a year (which apparently wasn't enough even though his wife Matilda said his court was pretty lavish for anyone who wasn't a King or an Emperor and reminded him again why her first husband was better since comparing her husbands was something she always did with Geoffrey and her father). As a co-king, all contemporarily chroniclers point out he had expenses to meet that a normal crown prince wouldn't have or he would lose face among the Anglo-Norman nobility and even visitors from nearby places like the HRE.
Henry the Young King plotted rebellion with the Anglo-Norman nobility, Breton Magnates, his brothers Richard, Geoffrey, his mother Eleanor, the powerful Philip Count of Flanders, and King Louis VII. Eleanor and Louis often get the blame for being the initiators of the conspiracy. Phillip brought in contact with William of Scotland. Maybe Henry the Young King got cold feet at first, but he didn't act for awhile. He only acted when Henry II promised three key castles earmarked for Henry to John.
In 1173, the entire Anglo-Norman realm went into a short civil war. On the Continent, so many nobles had deserted the old king that he found the defenders of his French possessions outnumbered three to one against Louis's men and the rebel Normans alone. Despite his numerical disadvantage, his commanders starved off defeat and even retook some castles. Henry quickly crushed the revolt in England and captured the King of Scotland. Despite the Normandy front holding, he spent a fourth of the treasury hiring some 20,000 mercenaries (ironically mostly Flemish) to bolster his loyalists and by the next year, everything was back under control.
Eleanor ended up under house arrest. Aquitaine was given control to some Normans Henry appointed to rule in his wife's name (later Richard would run it). Louis paid something. William King of Scotland was forced to pay homage to Henry (although Henry never bothered to seize control of Scotland, so this was all fiction). Henry forgave his sons and hoped it would never happen again. As it turned out, the Plantagenets were not immune to family feuds.
But suppose the course of the war went differently. I got inspired by the events that allowed the Old King's followers to retake a castle in Brittany. Basically, 30 of them rode straight in an open gate late in the afternoon with their banners in plain sight. After less than an hour of fighting, they convinced the 90 defenders to surrender. Whoever was assigned to the gate should be fired.
Suppose Phillip Augustus, son of Louis VII, gets stationed in a castle closer to the Norman Vixen at the start of the revolt. At the beginning, most of Henry II's followers play a defensive game. However, a minority of them do a Chevauchée into Capet territory. A group of them find a castle with its gate just lying open stupidly. They go in, and overpower the defenders. They grab any portable valuables and take young Phillip (he's 8) hostage.
They bring Phillip back to Rouen and treat him with all the respect a noble captive should have. Actually, at this time Rouen is more impressive than the Capet court according to many contemporaries. one mention is how the Normans seem to be able to afford as much wax for their candles as they want, something that isn't as common in 1100s as it would be 100 years later. Henry II orders a bishop and two not oldest sons noblemen as his caretakers and try to form a bond with him. He also orders anything the boy wants to eat, he can have. The war in England plays out like in OTL.
When England is secure, Henry II tries to end the rebellion in Normandy without mercenaries at first. He reasons his own forces held out for awhile, so they just need a little extra oomph. He orders a mobilization in England and lands with knights from his trusted Anglo-Norman vassals as well as 25,000 levied men armed with his own expenses.
However, take castles is not the same as holding them and the levied men, despite being given good quality gloves, spears, and shields, fail to be as effective as knights (oops). Some ground is retaken and some skirmishes are won, but the war isn't over by 1174. Meanwhile, some nobles in England are wondering when they are going to get their farmers back, because they kind of missed out on one season's worth of harvest and would like them back for the next season. The levies are sent back in time for the planting in 1175.
With the war still going on in 1176, some are worried that the old days of The Anarchy are going to return (Normandy suffered a lot less than England since Geoffrey conquered it ending the stalemate, but it wasn't a pretty time for them either). Eleanor of Aquitaine is captured.
In 1177, Henry II hires some 3,000 mercenaries for the final push. He ends the siege of one of his castles and captures Phillip of Flanders. By the end of the year, the rebellion is over. Since he hired less mercenaries, even though the war was longer he spent less money than in OTL. However, the longer war caused some disruption in Channel Trade and pirates took advantage of the distraction like they did in The Anarchy until Geoffrey finished his conquest.
William, son of Henry the Young King and Marguerite, is born and dies on schedule.
The peace is very similar to OTL. William of Scotland pays homage to Henry II. Louis gets the same terms as OTL. Phillip of Flanders gets off with nothing more than offering an apology for invading Normandy (in OTL he didn't get captured and got away with no gain or loss). Henry forgives his sons. Eleanor is in house arrest and Henry II appoints Norman administrators to rule in her name, I guess like in OTL he considers her the mastermind of it all.
Louis agrees that he will pre-emptily approve of any marriages Henry's son might have with nobles in the Kingdom of France. Any exceptions are duchesses, and any descendants of Henri I of France, which may be approved at a future date, but is not being pre-emptivly approved in 1177.
Henry knows that part of the tension with his overlord, Louis, is that the birth of Henry's sons displaced Eleanor and Louis's daughters from Aquitaine. Henry decides to try to diffuse some tensions. Acting in the name of his wife, he offers to let Eleanor's oldest daughter inherit the Eastern 1/8 of Aquitaine.
Phillip Augustus spent slightly over three years in luxurious captivity. His father is allowed to pick him up. He has a dinner in a Vixen castle with his son and they spend three days together. However when Louis goes to pick up his son, Phillip says to his face "Dad, I want to stay with uncles (insert name of the three cargives) and Aunts (insert names of sisters of caregivers), can you come again next year?" Phillip is quickly rushed out of the castle, separated from father, and the Normans say "see my liege? He wishes to spend the next year not in Paris. It's your son's wishes" and brush off all attempts for him to come.
Henry assigns Richard as a fourth companion to Phillip and tell him he is to keep the boy entertained. The Normans tell everyone the young Phillip is free to return to his immediate family whenever Phillip feels like it. When Richard is assigned to run Aquitaine, the land he is supposed to inherit, he takes Phillip and the three Normans Henry II assigned as companions.
So what are the next probable developments? Did Henry II neutralize the threat from Phillip by having him spend so many years with Richard? Or did he just supremely piss off his overlord by separating him from his son and Louis is going to plot again? Does the prolonged conflict make some of the Normans lose some faith in the Old King? What is Phillip Count of Flanders going to think of being off with a slap on the wrist? How does being captured at such a young age, and then being treated with all the great food in the world and the best sights of nature to see, would affect Phillip Augustus's personality? When it becomes apparent that Queen Margurite might have loss reproductive potential after William's death, will another family feud happen?