To me it all comes back to geography.
If a nation is continually having to defend itself you end up in a conservative, authoritarian armed camp. basically a military structure for survival.
If a nation isn't being attacked by outsiders internal factions rise up and demand power, ultimately ending up in democracies, rule of law, etc.
The problem for Eastern Europe is it is right next to the Asian Steppe. The source of violent men on little horses for thousands of years.
OTOH Western Europe is is at the end of the European penisula. As technology allows communications to improve nation states are going to happen minimising external conflicts.
So, to flip the OTL political situation you first need to eliminate the threat of the Steppe where every couple of hundred years a boom and bust cycle spits out a barbarian hoarde.
Then you have to find a way of getting the neighbors to play nice. Pick an Eastern European nation and it is going to have potentially hostile neighbors on all sides. Not like easilly defended Avalon. Maybe the only way is a hegemonic empire? But with limited navigable waterways you are going to have to wait for railways to make it work.
Meanwhile in the West you are essentially going to have to find ways of stopping them forming nation states. Given that places like Spain, Frace, and the UK have really convenient geographic borders that is going to be tough. You need to find some way of keeping those micro states fighting so the loyal lords never feel safe enough to pull stunts like the Magna Carta.