Harm

maybe you could add the ability to set traps

If you don't want to change your trees, you could as well just add a new sort of bushes which influence the sight (like in starcraft) and use both to create forests. These new bushes etc. could also be the place where new trees grow.

have you played starcraft 2? there they implemented such a vision blocker: http://img641.imageshack.us/i/screenshot2011041411330.jpg (http://img98.imageshack.us/i/screenshot2011041411331.jpg/ )but i agree with comp382 to decrease the sight and the speed (especially for horses and siege units) and the trees should also decrease the ability to create formations since they are physical objects. this feels more realistic than a hard block of sight. a balance would then be the ability to cut the wood. also had the idea, that maybe some units could climb on a tree and use ist as a watchtower and climb on cliffs ;-). To have no vision on cliffs is problematic as it really favours the one upon them. Then you would also have to add a possibility to get sight on them without being on them. a further sight should be the better effect.and i have thought of how sound can influence strategy. If there is a radius in which you can hear units of the enemy (some are louder than others) this would be quite cool, too ;-).

I would like the terrain to influence tactics. For example you could add some trees or bushes or something similar which blocks the sight of the player and cliffs could favour the player who is on them in long range combat. And if your army is passing a river the movement speed could be reduced.