Premise:
A more wise Napoleon I in 1810 reached an agreement with the Tzar and an appeasement with the British Empire.
How can be possible in XX century an European (continental) Federation of Napoleonists Monarchies?
And who would be his allied and adversaries?
I've posted a
scenario (which is waiting on the backburner to become a (collaborative) TL some merry day) about a Europe united under a successful Napoleon some time ago. Napoleon indeed needs to do a couple things in a wiser way, but forget appeasement of the Tzar and the Limeys, they are not going to accept Napoleonic hegemony in EUrope without being crushed on the battlefield. And forget keeping the patchwork mess of vassals that was the OTL Napoleonic hegemony. Germany and Italy need to be given national unity in order to get content with Napoleonic hegemony. And the Austrian and Prussian turncoats need to be uprooted.
Basically, I'm convinced that Napoleon could have been fully successful if he had done a few things:
A) Make an organic effort to win the loyalty of European nationalists to his cause for good. Wipe out the old states in Germany and Italy entirely, including Austria and Prussia, and rebuild them together with the Low Countries as unitary/federal states in imperial real union with France (instead of the OTL vassal patchwork mess), make Poland and Hungary independent vassals, sell yourself as the explicit heir to the Romans and Charlemagne and take care that loyal members of the local middle classes make up the administration outside France.
B) Annex Danemark to the Empire and win Sweden to your side for good by giving them your support to annex Norway and keep Finland. Full control of the Empire's borders makes domestic enforcement of the ConSys pretty airtight, and in the medium term British economy is not going to fare very well by having access to Russian markets alone in Europe. The ConSys was hitting them pretty bad in 1811-12.
C) Deal with Russia with a more intelligent strategy adequate to usual excellent Napoleonic standards. If the Tsar wants a rematch, as he is most likely going to in the end, do not invade the frozen endless steppes on a wild goose chase. Take a defensive strategic stance in Eastern Europe, close to your logistic bases in Poland and Hungary, and smash Russian army after Russian army with your strategic genius until Russia is exhausted and forced to beg for peace, its nobilty officer corps bled white. Sweden can be most likely persuaded to give them a second front in exchange for imperial support of their reannexation of the Baltic countries. After Russia is done with, use your undivided might to crush the Spanish insurgents and annex Iberia to the Empire, chasing Wellington in the sea with tail between his legs.
D) Wait until stubborn Britain comes to realize the futility of continuing the struggle without allies against a united Europe. Napoleon could try and give support to America in its own struggle against the British Empire, further weakening it and building up a strong ally.
European nationalism was in its formative stage during Napoleon's time, and it would have been quite easy to remold the loyalties of the budding middle classes and progressive landed elites to the ideal of united imperial Europe (and plenty of cultural and political precedents existed in the Romans and Charlemagne). With a prosperous, stable Empire, and Napoleon giving back some liberal reforms, as he planned to do in his late years, European middle classes would have flocked to imperial administration, making imperial rule unshakable after the defeat of Russia. Britain would be powerless to challenge this continental hegemony, which in due time can easily outbuild its naval supremacy.
America would be a natural ally, esp. if Napoleon supports its expansion in Canada and Latin America. Britain and Russia would remain its natural adversaries, remaining locked into a Cold War and colonial competition in Asia and Africa with the Empire throughout the 19th century.