In October 1216 King John of England died, this led to his rebelling Baron's to suddenly turning their loyalty from Prince Louis to Henry III of England. After a further year of warfare Louis was finally beat and relinquished the half of England he'd previously held. 7 years later Louis became King Louis VIII of France, he never forgave the Baron's of England though for changing their allegiance so rapidly.
However what-if King John hadn't died and instead he survived to be beaten by Prince Louis? Obviously the Prince would become King Louis I of England as I doubt after gaining that much power the Baron's would then go against him anyway, however to make it simple let's say John's son Henry dies of a childhood disease through butterflies. With King Louis I of England how would we see relations between Louis and his father King Philip II of France develop? Would Louis cede the various Duchies to his father in order to secure the French throne as well rather than risk his father giving the throne to another family member (likely his younger brother Philip whom would be 22 by 1223).
What would Anglo-Scottish relations be like? King Alexander II of Scotland paid homage to Louis in 1216 thus might we see Louis rewarding him with a slip of land in the North of England? Also how would Louis interact with the various Welsh Kingdoms, I expect he'd try and keep the King of Gwynedd happy seeing as he was the most powerful and Louis would need many allies to help him in case of rebellion.
Let's say in 1223 King Louis I of England also becomes Louis VIII of France, let's say that for that moment in time all is silent in both England and France maybe using a bit of handwavium or simply good diplomacy from King Louis I & VIII. However in 1227* some dissatisfied English nobles rebel in favour of the 18 year old Richard, son of previous King John I of England. This 'Second Baron's Revolt' is put down by King Louis VIII whom wishes to keep his English holdings however he needs the help of Welsh and Scottish troops, for this help he hands out minor portions of land to these monarchs.**
Thus with King Louis I beating off any major opposition for the time being he's secured as King of England. However in order to secure his throne in England he marries his eldest son Prince Louis to the English Princess and daughter of previous King John, Isabella whom IOTL married the Holy Roman Emperor in 1235.
What do you think? Plausible with a small-amount of handwavium? Or is that not even required? Should I make a TL of this?
*Although IOTL Louis VIII died in 1226 ITTL butterflies lead to him not suffering from dysentery.
**This is a point ITTL I was thinking of having another kind of 'Harrying of the North' just maybe in Southern England. With hundreds-of-thousands dead we could see an influx of French settlers in order to 'Franco-fy' Southern England and thus create and Anglo-French England. However I haven't included this in the bulk as it was only a small thought for a long lasting Anglo-French Union.