As I mentioned previously, I'm working on the overland encounters code for HA!
Basically, if you end up having an encounter on the overland map, you are brought into a tactical map that is representative of the terrain type you are standing on at the time of the encounter. If you're on a road tile, for example, the zoomed in map will feature a road, grass and trees.
Combat occurs just like in the dungeon, except there are no walls, just a big open terrain map. Getting to the edge of the tactical map will return you to the main overlap map, if all monsters are slain. If monsters still exist on the tactical map, you will be given the option to attempt fleeing, but may or may not be successful.
None of this is exceptionally difficult to implement. What I'm struggling with at the moment is how to actually trigger the random encounters. I could take the easy way out and just do a random check each turn (with various racial, class, skill and magic modifiers) or I can actually have encounters moving around the map (unseen by player) and triggering an encounter whenever the hero steps into the same space as an encounter.
I like the concept behind option 2, because having entities travelling around the map makes it feel like more of a world to me as a developer. The problem is, it's a lot more work than option 1 and I really doubt the player will notice the difference. If I just have occasional random encounters, based on an arbitrary (but modified) number, it's probably good enough.
It seems silly to write extra code just to make it more interesting for me as a developer. On the other hand, why would I do it if I didn't find it interesting?
I'm curious how other games (and developers) implement this sort of thing... Comments encouraged.