I like charts and written rules to be kept to a minimum. I prefer to let common sense and a few written guidelines help players work things out to mutual satisfaction.
That said a few things came up regarding terrain in todays slaughter of the wild elves by the bat-clan goblins. The terrain was a pair of double-size hills and a regular size swamp. It seemed odd that archers on the hills could shoot freely at any unit on the hill (in the terrain) regardless of where the hill crest was. It also seemed odd that a unit on the hill top could not use this vantage to shoot over units on the flat. It also seemed odd that movement both up, down and across the hill should be 1/2 speed.
The swampy marsh seemed odd in blocking line of sight, although half speed movement and fighting penalties made sense.
Also it felt like flying units (such as bat riders, or flying demons, dragons etc) should be able to see and shoot over terrain and units on the ground.
Mobile cavalry being able to dash through the marsh at full speed was fine, but might seem odd with lava.
I was going to suggest the inclusion of terrain with effects on the three axes of Movement, Vision (including shooting), and Combat.\
I also thought about including "Impassible" as a terrain type, that could be crossed only by units with Mobility, who do so at 1/2 rate.
A swamp could be defined as affecting movement and combat but not obstructing line of sight. A hill would block line of sight, but might not effect movement or combat. A thick forest would effect all three.