Im quite new on the DOF front so bear with me if i suggest something which may already be part of the game...
As your castle becomes more developed i think it should be an option to build outposts for both trade and defense, this could lead to more interesting merging of the combat and trade/economy system due to the implications of having an trade outpost raided. It also increases game playability because it creates a more interesting skirmish/defend hybrid where smaller task forces could clash.
I feel that this is something which is often overlooked in games which seek to have a empire building style and would make for much more interesting economic gameplay
There could also be raids on caravan routes however this may be harder to implement due to the nature of the economy continuing while a player is offline and the combat not. although it could be interesting to have dedicated units who defend trade routes between different kingdoms ect
just a couple of suggestions
im open to any positive/ negative critism and due to being new if anyone could enlighten me on the diversity of combat scenarios, that would be great