I like your idea for a forested board, and one thing you might consider is making the board somewhat 'modular'. If you check out my recent battle report, you will see what I mean: I've made several 1' x 1' flooring tiles into a few different types of terrain. Not saying that's the best plan, but it seems to work for us.
This allows us to remake the board as we desire, changing things to represent different types of terrain, or a defence position etc. Plans are to make at least 3 times the number of tiles that you see in the BR - when done, we'll have mountains and rivers, more plateaus, maybe a few buildings. Likely a lot of buildings.
This still lets you place smaller terrain pieces on top of the board as well - forested areas, etc.
You're going to have a Chaos Temple, right?