1. The weapons are too good.
In the middle age, More troops are equiped simply. Most of the infantry didn't have armour. Of course,some elite troops had hauberk or Plate , but They were in the minority.To be the same as the real history, I think the advanced branches should be more expense and players should pay more to keep them.
2.troops should need food.
In the game , a troop can conque the world without supply. a troop can run 1000 kilometres in enemy's territory.But in real world , this is impossible. Actually ,Song won most of the battle with Mongolia, but Song can't conque Mongolia. Beause the distant steppe is barren, Song's supply can't reach the distant steppe. But can get food from the rich city which it conqued.
3.In real middle age, if An army is far from its motherland, The morale of the forces will sank to a very low level. Only if the soldiers get enough fortune for this expedition.
4、Government Efficency.
terrain of China is too lever, so the Song and Jing kept a big Empire with a very low Govement Efficecy. people of Song is rich, the Government of Song can't convert the people's wealth to government receipts, beause the country is too big and the government did't have highway and telegraph to manage it.
A city's efficiency will be lower if it's farther from the capital. A city's efiiiciency should will higher if it had more dock and road. A city's efficency will be higher if the tax rate is low.
With a low effiicency, the tax income will be low, and government should pay more to build communal facilities.
A city with very low efficency will appear Rebels.