On the rpg elements, your soldiers can swim through rivers, it just takes stamina to do it.
I would like it if you could get a few items and armor for you hero that would give stats, the odd ability, and a change in the model to reflect you hero wearing it.
For border towns, I would like an area where you can create another castle, for a lot of resources or some other prerequisite that makes it hard to get one, that would be capturable by other players. There could even be possibilities for guild warfare in this area by having guild cities that are cheaper than player cities, but aren't under any one person's control. There would be a "lord" appointed for the city, but other members could be able to trade there and recuperate with their armies there.
Another way to implement the idea is with various capturable objectives throughout the world.
Picture made by Aametherar the orc (Elves are the best!)