FH is already a great game. It is by far the best game I have played in years. Stardock is to be congratulated for the success of all of their hard work.
No matter how good there is always the potential to get even better. Here IMHO are the things that I can think of that would add to the game.
1. Stop repeated incursionis into the players territory by the AI when not at war. (The reverse player into AI or AI into other AI would have to be the same.)
This could work one of two ways (I am sure there are others too):
a. The first time you tell the AI to get out of your territory this would prohibit them from trying again for X turns.
b. Better would be the AI could could only enter the players territory when not at war if a specific treaty ("free passage treaty" ?) had been signed between the two.
2 Melee Weapon upgrades. At this point there seem to be two paths: one handed weapons and two handed weapons.
That causes too many unwanted changes of weapon types i.e. spear to two handed axe.
I suggest there be several upgrade paths: swords, spears, axes, hammers, etc That is upgrade by type.