Am I correct that curently all terrain has the same 3 effects?

Should buildings, ie castles, be counted as regular terrain? Or will there be special rules for these in future?

What about water areas? I could see a case to be made that large, deep bodies of water (rivers, lakes) should be impassible to most troop types, and should not block line of sight. This would however add an extra level of complexity.

I ask because terrain modeling is one of my favorite parts of the hobby, and in a fantasy game I can envision terrain types that would be a problem for some troops but not others. demons for example might be quite happy in rivers of flame or acid, flying troops would ignore cliffs that can be scaled by spiders and spider riders, but are otherwise impassible, etc.

I would be most happy if these were noted as options, to be agree upon by players before the game, not used as an excuse to add lots of extra rules and special cases.

The simplicity of MA is one of its chief strong points to me.

I do like the idea of different effects for different types of terrain.  I think something like what you designed should be included in an Advanced Rules book.  It does add more complexity, but it also adds to the realism.

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.