Only using standard doodads as in blizzard ones? If yes then
-Fog and weather effects would do it, and with any trees of your choice. Dark, greyish fog + a weather effect heavy rain or light which one you prefer more.
-A lots of shrubs and rocks with wise hight variation. A lots of trees, I would prefer the village ones or the dead ones, it's also nice to make them bigger then average and some smaller trees around the big tree/s.
-Adding water is an important thing if you want a stormy forest, I would prefer you to use a water model but if your just aiming a map without imports then lower the ground and you'll hopefully see water.
-Making ruins is also a good idea, old but wise constructions which the player can explore!
Note: making him only walk through a forest isn't really creative.
Hope it helped. Good luck.