Cool looking map. I'd certainly try a flat wash of color or tint for the forests, with or without a subtle pattern on it (could indicate type of forest). For a soft transition you could feather/blur the edge, or maybe break the large mass into smaller fragments and fingers in the valleys... not sure what will work best for your style. Take a look at some forest or region vegetation maps for more ideas, but I'd say you're thinking in the right direction. On another note, the label for "The Sarinian Islands" could perhpas use a little more breathing room.