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Challenge: Make it so the Holy Alliance of the post-Napoleonic era transforms into a reactionary pan-European entity intent on preserving monarchical power, aristocratic privilege, etc. against revolutionaries.
There'd be French troops policing Germany, German troops policing Spain, etc. to suppress any risings against the kings.
Bonus points if national identity fades totally into social class and we get a pan-European aristocratic consciousness crossing national boundaries. I think that would require more integration of the Old Regime states than just "all of us will help the other against revolutionaries."